Sharing food from different dishes

Hello, everyone!

I have read many times that in Japan it is frowned upon to order only one "meal set" and share it between two people. But I wanted to ask: if we order two meals (eg.: two ramen bowls, two sushi sets, two orders of fluffy pancakes) is it rude to eat half of it and then switch so you can taste both dishes? Or to take a little bit from your parners plate so you can taste what they ordered?

well, my husband (who is japanese) and I do this all the time. we often buy 2 dishes and share them. not had any complaints or funny looks.
i think its ok not to worry yourself with trivial things like this when you are traveling, no one is going to be watching you to make sure you are not breaking social rules, they are all enjoying their own food and spending time with their own loved ones to care about what you are doing. just have a good time.
my japanese husband has taken my plate and eaten all my leftovers before, i dont think its an issue.

Actually in a lot of places you do not order your own dish, but small dishes to share with everyone. Eg in an izakaya you order different dishes and then everyone gets a small empty plate and takes from the ordered dishes what you want. This is also the traditional way, Japanese eat at home. A lot of small dishes , shared among everyone.
The same is true in Yakiniku places where you cook at your table your own food on a shared hot stone/iron plate.

Nope, no problem at all with two people swapping dishes or taking bites from each other's plates. My wife (who is Japanese) and I do this pretty much every single time we eat out.

「I have read many times that in Japan it is frowned upon to order only one "meal set" and share it between two people.」

This is true, but the reason is economic, not etiquette-based. Japanese people don't think it's rude for two people to share food, they think it's impolite for two people to go to a restaurant and order only one person's worth of food. This is an especially big concern as Japan has many restaurants with very small interiors. It's not unusual to see casua restaurants (ramen, teishoku, etc.) with less than a dozen seats in total, so if two people sit down, taking up two seats, and only order one meal, it's an imposition on the restaurant, since they don't have that many seats to spare.

The same generally goes for cafes too. In the U.S., for example, it's not that uncommon for two friends to go into a Starbucks, only one of them to order a drink, and then both of them to sit at a table while the one friend drinks it. In Japan, though, it's generally expected that each person will order at least one item. There's nothing wrong with spending a long time to linger and relaxing as you drink your drink, but just hanging out with a friend without ordering anything is frowned upon.

As suggested, I believe the OP misunderstands the situation.

It's totally okay and normal for restaurant customers to share food. The thing that is frowned upon is for a multiple number of people to go in a restaurant and only order 1 set meal designed for 1 person. I think this is a universal rule.

So, for example, 2 people can go in a restaurant in Japan, order 1 set meal designed for 1 person, and then order 1 drink for the other person, and ask for an extra plate for the 2 people to share. Of course, some restaurants may have different policies, but it's totally normal to order like that if you don't know their policy yet.

Sharing is fine vetween people.

Ordering one meal but two people using two seats is frowned upon.

Sharing food from different dishes/swapping dishes is fine; two people ordering only one dish meant for one (while occupying space for two) and sharing it is frowned upon. I believe I mentioned this incident before on a similar thread:
A few years ago, at a "soba" (buckwheat noodle) stand restaurant inside a Shinkansen station, I saw a group of 3 apparently non-Japanese travelers walk in, bought just one large serving of "soba" noodle, which the other two tried, and hang around at the counter seating (though this was a bit spacious than usual - the place had more than just one straight counter at which everyone ate) sipping bottled water and chatting. This is a no-no. Had the place been a bit more crowded, I would have said something to them.

In the same way, in “izakaya,” you are expected to order at least one drink & one food dish per person, even to share. It is not good manners for two people walking in, order one beer and the other asks for (free) water, and eats a bit. Each person is expected to order a drink from the menu, be it beer or oolong tea/orange juice, and a food item.

If you walk into a restaurant during lunch time, they will want to know “o-shokuji desu ka?” meaning, are you eating, meaning, are you eating lunch (as befits lunch time). If you say no, you are just here for coffee, they might decline you (because it is busy lunch time), or they might be OK.

It is those unwritten manners/policy (but understood among locals) that some proprietors cannot explain (and cannot argue back if challenged) that the restaurants/cafes might have difficulty with, and also where the travelers can have difficulty with.