I believe I've read that Justinian actually did consider this at one point, before ultimately deciding against it - so it's entirely possible that he could end up going through with the plan in the ATL.
On paper, it actually looks like a good solution - it would allow the Eastern Empire to focus more of its attention upon the Persian frontier and free up manpower. There are some problems though, that would need to be dealt with - Italy had largely been devestated by this point and would need time to recover (though the presence of a healthy and stable North Africa would no doubt help in this), if the East pulls it's soldiers back, the West is going to have to build an army piecemeal and fast. It's not a sure thing that, with different circumstances, the Lombards would choose to emigrate elsewhere (or do so in a different manner), but Italy is still surrounded by potential hazards and will need to be able to defend itself. Finally, there's the issue of if Justinian's successors would honor the arrangement - late classical Roman history was full of tales of Eastern and Western Emperors conspiring against one another and seeking to reunite the East and the West.
Having said that - a renewed Western Empire, with an Emperor ruling locally and able to quickly respond to threats, does seem to have a few things going for it that Italy did not have in OTL during this same period.