You can mix tangs.To avoid and minimalize aggression.
A)Add them at the same time
B)I seen tangs of the same family mix just fine,other times I seen them fight viciously.The safer way is pic from different families,say one from the zebrasoma and one from acanthurus family.
C)Make sure they are similar in size.Don't add a 3'' regal with a 6'' yellow,fighting could insure.
The yellow(zebrasoma) and blue(Paracanthurus) should work.I have a desjardini(zebrasoma desjardini) and Powder Brown(acanthurus japonicus),they are best of buddies,inseparable really..Both were added at the same time and both were the same size.
Good luck,as with all fish,there is no quarantee.