Where a studio allows it, add every service you want to the same booking. The times you are then offered account for the whole visit rather than for one part of it, and the studio sees the full picture on its schedule.
Booking two separate appointments back to back is the thing to avoid. Anything that overruns pushes the second one, and if the two were booked with different professionals in the same studio, you can end up double-booked with yourself.
Some combinations cannot be booked online at all — a colour that needs a strand test, two services that need different rooms, anything that runs past closing. Where that is the case the studio usually says so on its page. Message them and they will build the visit for you.
Practical point on money: a long combined visit is where deposits are most common, and where the deposit is largest. That is not a studio being awkward. A four-hour slot lost to a no-show is most of their day.