sheik is easy, uptilt, upair, pound, if the misspace an aerial on your sheild, edgehog and then punish landing lag, if you crouch you can rest her jab, grab, ftilt, uptilt(i think), dash attack poorly spaced, dash attack oos if poorly spaced, i have heard u can rest a misspaced dwnsmash but i dunno about that one, probably crouch canceled fsmash, if she transforms into zelda....
marth is hard, uptilt and upair are weird, im not really sure exactly when it will work, i have just gotten a feel for it, i think it has to do with him not having a jump and stuff, fallin upair is alot better, edgehog to rest the lag, poorly spaced fsmash oos, poorly spaced dsmash oos, if you crouch, if he grabs or uptilts you rest
Also there are the general ones that work on everyone, dwntrhow can lead to platform techchase rests on both, nair rest and dair rest work on both, but i really like it against sheik because she cant punish well if you miss and if you mess up u still might get the rest if you hold down (you didnt combo the nair to the rest, but you autocanceled the nair and are crouching and then she will usually do something on the list above)
This is not a complete list, i just gave mostly the ones i thought were useful (i mean technically you can rest both of their taunts....)
-Mahone