Connecting UPS inverters in parallel
You don’t tell what UPS inverter you have, but I don’t think many models (if any) are able to be phase-synchronised. And if they’re not, you should not connect them in parallel : things won’t work fine as soon as they are out of phase, and you may even damage them if they lack protection. And even if they were phase-synchronise, you’d need to be sure that the current will be balanced between both UPS inverters.

If you have several devices to power, you may run two independent circuits (one connected to each UPS inverter). If you can’t, I’d suggest you buy a 4kVA inverter and ask your supplier to buy your 2kVA inverter back.