a quick question

is it possible to add two wads into a wad that is being edited? for example, i open up Doom 2 and want to use the guns from silverbullet and monsters from another wad, can i open Doom 2 and use both of these wads at once?

You're talking about two different things here...you can load multiple wads when you start doom2, but you can also merge them using a program like WinTex or what have you (there's a list right off the main doomworld page). That is to say, you could open your wads from the command line as doom2.exe -file this1.wad that2.wad.

what i meant was could i open 2 wads along with Doom 2 in Doom Builder at the same time. and i have tried Wintex and XWE, but neither of them will merge the two wads that i want merged into one.

Looks like Doom Builder only allows one add-on wad. If it's textures and/or flats I'd suggest exporting them to a temporary directory and telling Doom Builder where to find them. Sprites are a different story - have you tried export-import to transfer them into the wad you're creating?

to be honest, i didn't even know you could do that, so no, i haven't.