Didn't we just do the TPE thing with the trade for Bol and Dozier? They came in to the Celtics with the Kemba Walker TPE so didn't count as incoming salary. Our outgoing salary was JHern at $7.1M. So even though we took in about $4M of salary and sent out $7.1M, we were able to end up with a new TPE at $7.1M and also extend the date when it expires relative to the date that Kemba's expires.
We could do exactly the same thing with Horford only the numbers are bigger and we would need to use more than 1 TPE to cover the incoming players/contracts. It both consolidates a couple to TPE into 1 TPE, which is good as you can't combine them directly, and also kind of rolls them over and extends when they expire. The tricky part with Horford is that the incoming salaries need to match up with our individual TPEs to get the most out of them.