Indeed, it is not clear what you mean by "We want the same dialing plan for both".
You should start by connecting the two PBX systems using a SIP trunk. This trunk should be used for outbound routes to the other system.
Once that is done, you could configure both systems to use an identical numbering plan - the same extension range(s), the same way to make outbound calls, the same ring groups, etc. - to dial extensions on the other system, you'd need to use a prefix in the outbound route.
If you actually meant that you'd like to use a "coordinated dialing plan" on both systems, that can be done, too (at least on the UCx side - but likely should be also possible on the other IP PBX). If the extension ranges on the two systems don't overlap, all you'd have to do is to configure the outbound route with the other system's extension range.