Just treat and report each unit as a physically separate structure. You just split the mortgage interest, property taxes and insurance between the two. Once all the rebuilding is done it's perfectly possible and highly likely that your cost bases for one unit (the burned up one) will be higher than the cost basis for the other unit. Nothing wrong with that.
So for 2023 you have one unit that was rented out starting at some time in 2023. Treat that as a physically separate unit. Once rebuilding is completed on the other unit you'll enter that as a physically separate rental property with it's own cost basis and start depreciating that one the day you make it available for rent.
