That was very "punny" :P

In all seriousness there is a couple in the comments below.
here is a simple 7 rounds, uses an Esplosioner first round, swaps in level pet, then Baal and follow same steps (edited)

5 Star rating for the pun.

Interesting in theory but running through the numbers, I believe this is only viable if 1 of the level pets dies (Pesuming you are passing turn 1 in order for that pet to actually get the experience)
If you are swapping straight to your 2nd level pet turn 1, your 1st won't get the XP but as long as only 1/2 of hits on your Baa don't crit, you'll be fine.
This can work and will be 8 rounds but as mentioned AFAIK only your #2 pet would get XP.

Now it's been a hot minute so I have worked on this strat but I believe there is 2 worlds where you can do this with 2x level pets (Unless I am missing something so anyone please @me if you can).
I can help with scripts etc and we can get you up and running with this strat on here if you like :)

1. Have level pets tank Pump, it'll be 9 rounds, no chance of failure.
Use Baa first, Murder on turns 1, 5 and 9, swapping a level pet in to tank the Pumps, 1 round of trample required. You can use Mangiacadavere if the first level pet dies to be able to tank the 2nd pump, saving the 2nd leveling pet from taking any damage (this would still split the xp and half the one that died).

2. Have level pets tank Thundertongue, it'll be 10 rounds, <1% chance of failure.
Use Level first, Swap to Baa, Murder (turn 3), Trample, Swap to other level pet, Swap Back, Murder, Trample to victory, again this can work but only allows for about half the attacks against you to crit. Small chance but I must mention it.

I'd be more than happy to assist if you'd like to turn it to a level pet strat or discuss further :)

So far this has worked for me. I've done it about a dozen times and each time it is whatever leveling pets are up, some 1st, others higher. I have had both die and they get XP, and sometimes they live. I engage, swap out 1st for 2nd, then onto Baa. He is low in heath but lives. Both level pets get XP.

It might work but is not optimal and guaranteed:

Here is the 2 sims.
* With both as level 1's: 90%
* With both as level 24's: 5%

Now that is presuming you are passing round 1 (if round one you swap straight to pet 2, pet 1 would get no xp).
Now if you are attacking with your first pet, that might work depending on what you do but that then is out of the realm of strat making TBH as we can't predict that move

Now if you are swapping to your #2 pet straight away it will work BUT then your #1 pet won't get any experience, your #2 will take a tonne of damage and may die causing you to get only half xp.

Now if you really want to use Baa'lial; here is a simple explode first, 7 round strat with a level pet, just put an Esplosioner as #1 pet (sorry, the double level pet Baa'lial strats are above, i'll help you build them if you want):
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use(Explode:282) [round=1]
change(next) [!self(#3).active]
use(Murder the Innocent:2223)
use(Trample:377)

-- This script was generated with wow-petsim.com
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Edit: In case you didn't know, using 2x level pets halfs the experience gained even when both survive. This strat gives so little xp that 2x level 1 pets used in this way might not even gain enough to go up a level anyway.
If you used 2x level pets and they both died, then each would only get 1/4 of the max leveling experience on offer (edited)