So if I am interpreting this correctly then you are proposing to purchase a property for c. 150k and I assume you are keeping 10k aside for purchasing related expenses?
It makes sense on one level as the extra mortgage of 110k (on top of 210K from investment property) will probably not cost you anymore than the rent you are currently paying, but I guess it depends on how safe your jobs are and I'm sure (hope) that the banks will assess this side of things.
Lots of investors have gone interest only in the past but personally speaking I would continue to pay off the capital on your investment mortgage for as long as you can. If house prices rise then all the better, and you can treat the capital repayments as a kind of a saving scheme.