I made up some double tounging studies which help with the problem you're talking about. Many of them never made it into my books.....but this one did, in one of it's variations.
What I do is find the note just one half step above the note which you have trouble double tonguing on. Let's say low B flat gives you trouble, then you should start this exercise using B natural or C. The idea behind the exercise is to start with a simple TaKaTahhh and expand that into a long sequence of TaKaTaKaTaKaTaKa........etc.
Here's an abreviated version of the exercise:
||: Ta Ka Taaaaaah :|| rep. 10 X's (breath as neccesarry)
||: Ta Ka Ta Ka Taaaaaaaaah :|| rep. 10 X's
||: Ta Ka Ta Ka Ta Ka Taaaaaaaaaaah :|| rep. 10 X's
||: Ta Ka Ta Ka Ta Ka Ta Ka Taaaaaaaaaaah :|| 10 X's
||: Ta Ka Ta Ka Ta Ka Ta Ka Ta Ka Taaaaaaaaaah :|| 10x's
Get the picture?
Once you've done this on C, do it again on B, then on B flat, A, Ab, G and F#.
The same thing can be done with triple tonguing.
It's very important that the air you use for this exercise, and any other double tonguing exercises for that matter, should be sustained and smooth.....as if you were playing long tones.....not a double tounguing study.
Eddie "Tiger" Lewis