You can randomly generate a number between 1-2 and save it in a hidden question. You can use the function like SysRand or RandNum within the hidden question. So RandomNumber=1 is asking order 1 and RandomNumber=2 is asking order 2.
You can then set up your questions in the 2 different order sets and use RandomNumber in skips to jump the question set not relevant.
The disadvantage here is you have 4 questions (a1,b1,a2,b2) saved in 2 seperate questions each. You can combine the 2 in free format hidden questions later in your survey so you have a1tot, b1tot, a2tot, b2tot) or you can combine them in your analysis software.
This may not be the best answer but it will work.