this is a nearly impossible question to answer with the info you have given...
ill start with acid- tabs of acid vary considerably with each dose. one blotter could be enough for a decent trip or you might need as many as 3 to even feel it. id say start with 1, and if your not feeling much in an hour take another half tab. also, everyones tolerance is different, so if a friend says its weak acid, its possible that it could totally trip you out.
mescaline- are you talking about cactus here? if your getting what someone calls 'mescaline powder,' chances are your getting some sort of 2c-x. mescaline powder is extremely rare and hardly ever manufactured since other drugs are much easier and cheaper to produce a dose (production vs profit, all that). so if your talking about san pedro cactus, again it depends on how hard you want to trip, 6 inches is very mild, 12 inches is where you get into tripping territory and get a nice slow high and 18 inches is a very heady dose. of course you can go higher but as a first timer i wouldnt recommend it. and again, since this is a plant the amount of mescaline varies greatly.
if you are getting what is being called mescaline powder (and 95%chance you arnt), be careful, this either means the dealer is selling you something else on purpose or he really is ignorant enough to not know. doses of 2c-x are measured in the low mgs, so you can easily take to much and have a seriously bad trip. stay away from powders until your initiated a bit into the psychedelic world (acid, shrooms, mescaline from cacti), this is just my opinion though and im sure others would disagree.
psychedelics are not like other drugs, like pharms and such, with measured doses. basically, you are going on the word of the person who sold it to you or friends who have taken the same batch, to get an idea of the potency. but its kinda a guessing game, always start slow and work your way up. with experience you will be able to handle larger doses, but as a first timer start out slow.
you really should read up the big and dandy acid and mescaline threads on here and all the info on erowid... the more info you have the better...happy tripping
also, what do you mean by question 2? both have a cross tolerance with the other, so if you should wait at least a week between tripping. but if you meant at the same time then they would combine just fine, but if you have access to both id say do them seperately and experience both as their own.