Boyelectric, combi can sometimes be fitted alongside a cylinder but I have never seen it fitted to a thermal store. For a start the expansion vessel within the boiler would be grossly undersized.
And a combi fitted to 2" primaries? Come on be realistic. What with the system being open vented. Most combis are for sealed systems only. Some plumbers and a lot of homeowners think combi boiler is the only one marketed. When a boiler is fitted, clients needs and best options should decide what should be fitted. In a lot of cases combi is totally unsuitable.
OP. The plumber may have fitted a cylinder that looks slightly bigger than the older cylinder but due to rapid recovery may 'act' as a larger cylinder. When you talk about cylinder being capable of sourcing two showers, I have a client with 300l cylinder that is 'emptied' by his two sons when they shower. Cylinder soon recovers once the boys are done and the old man has had his tea and toast.
Free estimates. Stopped doing these years ago. Verbal on the phone suffices (site unseen). While client should ask for two or three estimates/ quotes, the exercise is pointless if spec is not the same. If I am going to supply and fit a top of the range boiler with copper plumbing, pulling most of the bends, fitting controls for economy and making sure clearances and access is good for future repairs and servicing not forgetting proper system cleanse and treatment, no way my price is going to be accepted against a heinz beans combi from the sheds with spaghetti plastic plumbing and no system treatment.
If the height of cold water storage cistern from top of shower spray head is small, regardless of size of cylinder, you will only get a dribble from the shower. This is regardless of quality of plumbing.
If you are looking to have good showers, suggest you get an unvented cylinder which will answer your prayers subject to mains pressure being adequate.