Why is your wife not insured for the house mortgage amount?
You are saving on two fronts, all capital repayments on the house and investment property.
You can consider the investment property as your pension if you want.
Both properties in positive equity, and both affordable. Good situation compared to many who post on here. It's hard to have money at the beginning, when you start out with a mortgage, and have two young kids etc.
To increase your savings you have to cut back, if that's possible. When you post the figures up we might have some ideas. Be honest !
I presume you are all tax compliant on the rental, you don't want that to come back and bite you.