How do I split a jointly filed tax debt so as to avoid collection but not have to take on full responsibility of the debt?

When you say that you are legally separated do you mean that you have a separation agreement that is in place or that the separation was ordered in court proceeding?  Certain states allow both and how this was done to begin with may govern how you should proceed here.  If you and your spouse signed an legal separation agreement which also distributed your assets then you should consider having an amended agreement prepared to incorporate this new debt and split it 50/50.  Be aware that the IRS does not care about your agreement.  You are both jointly and severally liable for it.  It just gives you an avenue to sue your spouse.  If the separation was done by a judge then you may have to go back to court.  Either way seeking legal help in your area is a good idea.  Good luck.