Hi,
We're combining 737NG/Max and 787 at TUI Belgium yes.
In practice, roster is completely mixed, you can fly a 737 sector on day 1 of the week, then depart on a 4 days 787 mission on day 2. On average, 1 or 2 long hauls per month, the rest is filled with 737 flights.
Very nice to be able to fly both as they both are very nice to fly ! The 787 for its comfort and the 737 for the fun of flying a lot on nice destinations. But it requires more work to keep updated on both types, reading books of both aircraft at the same time, and try to not mix knowledge of one onto the other.