As others said, anything works in friction mode. The XT derailler serves an 11-34 cassette without problems, although sometimes the derailleur hanger may be a little bit too high for optimal use. For example, the derailleur on my 25" Trek 520 handles a 34 cog beautifully and could probably handle a 36; however the same derailleur on my Co-Motion tandem (18" rear end) just barely handles a 34 cog because the hanger is a bit too close to the axle.
As for handling a 46/34 combo, I assume you mean a 46 chainring, right? Well, the rear derailleur in itself can handle any chainring you care to install, be it a tiny 22 or a large 64. The limitation comes in how wide an interval you want to use it through, and it's around 47 teeth for the XT. The # of teeth it can handle is usually stated somewhere in the derailleur specs. For instance, with an 11-34 cassette and a 44-34-22 crankset, you have 23 teeth (rear) + 22 teeth (front), for a total of 45 teeth.
However, if, for example, your chain was cut for 46-36-26 chainrings and a 11-30 cassette and you want to install a larger cassette (11-34 for example), you may need to lenghten the chain by 3-4 links. Anyway, while on the workstand, make sure the chain is long enough to handle the large-large combo plus one extra link. Even if you never use that combo (you shouldn't anyway), it's nice to know you won't break your bike if you do so.