Would you qualify for a mortgage for the 300k you need to trade up? Drawdown mortgage (maybe even get cashback), and repay it once the apartment sale closes. It'd also out you in a lot stronger position purchasing, as your funds aren't dependent on your sale.
It's difficult to manage closing both sales the same day as there are lots of variables. It's possible, but can be stressful.
Another option is to sell you apartment to an investor, and rent it back short term until your purchase closes.