If you use the gateways headless - i.e. using the open source components - this is possible with no problem.
But if you use the dashboard/portal/analytics, etc, you will need a licence to cover the additional gateway, details can be found on the tyk.io site. Paid licences kick-in if you use 2 or more gateways, the free version is only for a single gateway.
Tyk is designed so that all components are horizontally scalable, but there is a licence implication if using the dashboard.