I want to record iPad musicstudio instruments on my iPad GarageBand app. Can this be done?

I assume you mean Xewton MusicStudio app. Recording from any app into GarageBand is not possible right on the device. GarageBand does not have Audiobus support yet (and who knows if it ever will). You can always use audiocopy.

You can't record into GarageBand from MusicStudio, all you can do is "transfer" tracks to GarageBand with audiocopy.

For example, make a new project in MusicStudio and record an instrument. Now save it. Now tap the "Export" button and export it as a wav file (if you have more than one track in your MusicStudio song you can tap "Save tracks separately"). When you tap OK, the wavefiles are saved into the "Audio" folder you see in MusicStudio. Go into that folder (it might have already opened after you saved the wavefile) and select the wav you just saved. Now tap the "Share" button. Select Share via "Audiocopy" and tap OK. On the next screen tap "Copy Audio". Now tap "Done" (don't bother looking for GarageBand in the "Select app to AudioPaste" list that comes up...it's not in there). Close MusicStudo and open GarageBand. Create a new song. Select the microphone icon (Audio Recorder) for the track. Go to tracks view. Tap somewhere in the blank track and select "Paste".

That's it. The track from MusicStudio will be in GarageBand.

This may or may not be useful to you. It's nothing like recording an instrument along with your pre-recorded Garageband tracks (which you can't do from another app), but some people can make this work. Having the tempo and the time signature match in both MusicStudio and GarageBand helps.

Well, reading the manuals help... kind of 😉. The instructions weren't totally clear. I had to try a few different combinations of selecting and tapping, etc.!