Boss Br 900...line in

[glenn]

hi
I have a Br 900 recorder.I'm a drummer and want to record my e-drums into line -in and guitar /or Bass simultaneously.It seems I can record guitar -in and vocal -in simultaneously, but not guitar /line -in together.Bummer because I want to record the tracks live with rhythm and guitar.
must I just record drums first then add git and bass? Thanx for any help