Here is how it works.

自住房贷款25万，市价32万

投资房贷款2万，市价30万，贷款预计一年多付清，月贷与房租持平

Borrow the equity out of rental and use the money pay down the primary residence mortgage of $250k.  You want to see the results like the following.  Then wait for the opportunity to invest.  If you want to sell the rental, then do nothing.

自住房贷款2万，市价32万

投资房贷款25万，市价30万，