Hi,
If the two businesses as a Sole Trader and a Limited Company and clearly separate entities, then they would be treated as such and each have their own VAT registration threshold.
Guidance on registering for VAT can be read in notice 700/1here:
Who should register for VAT (VAT Notice 700/1)
The businesses may be looked at as one entity if HMRC feel they have been artificially separated to avoid VAT.
Guidance on when HMRC class businesses as a single entity can be read here:
VAT Single Entity and Disaggregation
Thankyou