Openreach then have a mechanism to match the two and do them on the same day.
Sort of. There is unfortunately a whole layer of abstraction with the whole process which is why a SIM in it's current form is required in the first place, explained below.
It sounds as though Zen just submitted two independent orders.
The phone and broadband orders will
be placed independently of each other.
For the CP (Zen in this case) the PSTN order will be placed directly with Openreach (which sort of isn't true in itself but a good simplification). The broadband order will be placed with BT Wholesale who then place an order on the CPs behalf with Openreach.
For SIM1 orders (what Zen appears to have used here) the PSTN generates an OMR (Order Matching Reference) that can then be submitted against the broadband order with BTW for magic to happen behind the scenes and allow the orders to match together.
The whole magic behind the scenes is automated and can, and does, fail leaving the broadband order to be discarded to be placed again as a stand alone once the PSTN order completes. There is no manual matching recovery process, leading to much hilarity and end user annoyance.
Sadly you don't get a single bundled order placed together at the order origin, with Openreach. This would be an infinitely simpler process but would require a complete re-work of the entire system leaving it a flight of fancy.
In theory Zen could scrub the PSTN order and re-place the orders again from scratch, hoping this time they match. However in your situation, while the whole process is going to take longer, shouldn't leave you high and dry without service. The PSTN should move over, and as it'll still be a BT line, your Plusnet broadband will carry on until it is transferred itself (unless they're putting you on LLU).
With the removal of MACs the whole process is in a bit of upheaval at the moment, a lot of peoples migration experiences already mentioned in this thread are no longer applicable, and especially with the introduction of SIM2... but that's a whole different beast!
Hope this helps to shed some light on your current situation.