I have stacked Trace 4024 inverters on my boat. Trace was bought by Xantrek and they are still selling and supporting such units. I do not know if your models have that functionality or not. I also think that "stacking" may serve a different purpose than you intend.
With one inverter you would get 1 KW of 120v power available. With two stacked inverters you would get 2 KW of 120/240 volt power. The stacked system enables serving a panel like those in our homes where two pole breakers give you 240 volts and single
pole breakers supply 120 volts. For your purpose you could simply add a second inverter and run additional 120 volt breakers from it.
If you are supplying a 120 volt panel from your inverter, you would be better served to replace your 1 KW unit with alarger one instead of adding a second unit. The second unit would have to supply a second panel. The inverter outputs can not be connected in parallel.