It really depends on what your travel & spending look like. Once you have FF miles in your account, you can get award tickets for anyone, for no additional cost. In this case, the ideal arrangement would be to have all your miles in one common account, where you collectively have enough miles to redeem for awards. British Airways has this in "household" accounts, were you can link 4 (I think) accounts together, and redeem from that pool.
If all of your mileage earning was via credit cards, then the better approach would be to get a card linked to one MP account, and spend on it. You can get additional cards for spouse/family members, for no additional charge, which also earn miles for that account.
For regular purchases, this works well. However, when you factor in signup bonuses, which can be 25k or more miles per person (account), one can make a good arguement that it's worth getting separate card for both / all people. Even if you keep the card for one year, the annual fee for the card ($50+) is usually well worth in, considering the 20k+ miles you get for it.
If you fly on the airline, each person earns their miles separately. If you don't travel far (thus not earning a lot of miles), it's not necessarily bad to "orphan" those miles in an account. However, if you take lots of trips, or longer ones (ie. East Coast to Hawaii is about 10k roundtrip), then you have an appreciable amount of miles. In this case, it might make more sense to get a separate credit card linked to that account, so you can earn enough miles to get that account to an amount sufficient to redeem an award.
Jeff