I presume both properties are in joint names?
The nominal negative equity is €250k.
In reality, the negative equity on the investment property is less because there is a cheap tracker. Without the figures, it's impossible to estimate this.
But let's say you have negative equity of €150k and the other has €50k.
This is what a fair solution should be...
1) He pays you €50k
2) Each property is put into your separate names
3) Each mortgage is put into your separate names
In practice, I doubt if this will happen
1) He probably doesn't have €50k
2) He will dispute that it's worth €50k for him to be released from the negative equity in your property.
3) The banks won't allow you to split the properites, as in effect, he is guaranteeing your mortgages and you are guaranteeing his.
4) He doesn't seem to want to cooperate.
As Bronte says, put up the figures for both properties - value, outstanding balance, interest rate and lender.
Brendan