Unless your finances improve the only sensible course is to sell one of the properties.
The mortgage lender for property 1 cannot force you to sell property 2. What they will do, if you fall into arrear, is commence possession proceedings. If you cannot pay instalments plus something monthly off the arrears an immediate order for possession will be made. The mortgage lender will sell. You will receive any surplus after the mortage and costs repaid. If a deficit you will be liable to pay.