i cannot imagine it would be a problem and have considered doing the same thing (since we get two rooms anyway, and last year w/the 5+2 deal, it cost us less to book separate 1A/1C per room/ressie)... if you ran into friends at the park and wanted to sit at the same table and have a meal, w/one family on meal plan and the other not, would it be a problem? probably not... so why half a family can be on and the other half not, i don't see it as a problem... you will just get a bill for the people w/out dining cards...
as far as sharing goes, once you order counter service and bring it to your table, i cannot imagine anyone would care who eats it (including whether you opt to feed it to the birds!) although at a sit-down, depending on the venue, you might incur a plate sharing charge... of course, you cannot "meal share" at buffets or all you care to eat restaurants...
good luck!