I suppose we'll need to have a second OR fibre line into the property if they decide in two years time to split the BT DV line.
...I will be splitting our services in 18 months time.
By "split the BT DV line", If you mean decouple the landline number that is bundled in with the BT package - such that it can be *ported* to another voice service provider for them to host the number for you, then no you don't need a separate fibre into the property.
Landline telephone numbers are no longer associated / tied with physical circuits and there is certainly no 1-to-1 relationship between them, as I think you may be implying.
You can in future port the number to another provider and run it over the one same fibre connection. It's just a VoIP service. There are caveats around the manner and timing in which your port numbers, especially in relation to a bundled broadband and voice agreement. Doing this generally terminates all the services within the agreement including the broadband.
However you could in theory and practise have dozens of VoIP enabled numbers running simultaneously over a single fibre broadband service. the bandwidth calls use is relatively small compared to what is available on the broadband connection overall.