Is it fine ordering 1 serving for 2 ppl?

Hello
Is the practice of ordering less dishes than the people present acceptable in Japan?
eg. i only ordered 1 beef bowl in yoshinoya but i occupied 2 seats together with 1 of my friend.
or a group of 6 people ordered only 4 main dish in a restaurant.

The idea is for sharing the dish together, presumably because the portion is to big for 1 person, or to maximize the culinary experience.

Thanks.

Really it depends on the type of restaurant. It's not polite to occupy two seats in a Yoshinoya if you're only ordering one dish. On the other hand if you're in a Denny's or somewhere like that with large main dishes, it's not that unusual to share them.

If you're in an izakaya, dishes are small and shareable, but it's customary to order at least one small dish per person, and everyone should order at least one drink, even if it's a soft drink, and pay the cover charge. (That's the system that keeps them from going bankrupt.)

I think that in any restaurant or cafe or bar, it's acceptable if each person orders something with a price. For example, if a whole meal is too much, you can order drinks (other than the free water or free tea) to cover the number of the group.

Then, if you see other people waiting for seats, it's common for the members of the group who have finished trying the meal or are already satisfied to go outside and wait while his/her friend finishes eating/drinking.

The idea of not wasting food is widely acknowledged as well as the idea of not occupying seats for too long. That said, Yoshinoya (and places like ramen/soba stands) is not really designed to be a place to chat. You're supposed to order, eat, and get out when you're done.

Just the other day, at a "soba" 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.

Right, AK. That's quite different from that "Ippai no Kakesoba" urban legend we used to hear about ;)

I guess i should ask the waitress before i ordered the food?
What should i say in japanese to ask whether its okay to share some food?

Just point at the menu and yourselves and ask "share okay?", because both words have been imported to the Japanese language.

But sharing is hardly a problem. It's "not ordering" that is. At least order a drink if you're not ordering food, unless you are a small child. And if you are worried about the ordering, just ask "amari taberaremasen. koredake demo iidesuka? (These are all we can eat. Is it alright?)"