As you may already be aware, Ta and Ti are both easily welded materials. The question of welding Ta to Ti is interesting. I'm wondering about the application for this combination, why do you need to join these two metals? Also, what exactly do you mean by "the welds sound brittle", did you actually perform some type of destructive test? With regard to the actual metallurgy of the Ta/Ti weld, hopefully you'll get a response from a metallurgist that has experience with this.
I suggest you to get in contact with the titanium great specialists, The Titanium Corp. of America:
www.timet.com
They're very kind people, willing to help those who need advice to weld titanium.
Giovanni S. Crisi
Sao Paulo - Brazil