Obviously if the two showers are used independently, each will work satisfactorily. If the showers are ever run together, and you must assume they could, it is more than likely there will be problems.
Certainly a 'proper' installation would see separate supplies from both the header tank (which should be increased in size) and the DHW cylinder. The feed from the header to the DHW cylinder should be increased from 22mm to 28mm too.
Unless you are using a Surrey flange to draw the hot, two showers could significantly increase the chance of sucking air into the hot feed.
If the existing feeds to the shower where 22mm it is possible you may get away with it, but I am not confident as I have never experienced it being done that way.
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