It does unfortunately happen where you can accommodate one and not both. Both employees here are either disabled or likely to be.
Employee A - her adjustments are in place. She has a lengthy period of service. I don’t think you can reasonably ask to change that, unless she is genuinely agreeable to a seated arrangement on the counter of some sort. If that isn’t possible, leave her be (but I do think you should explore the possibility to avoid prejudice to employee B).
Employee B - you need a medical or OH report on her condition to firstly assess whether she’s likely to be disabled, and secondly to assess whether there are any adjustments which are reasonable. Alternative duties MIGHT be reasonable (which is why I’d probe with A in a non forceful way whether she could do a counter job whilst seated).
If the OH report says she isn’t disabled, then no duty to make adjustments arises, so she’s either capable of doing the job or she isn’t.
If A can’t change her duties, and B IS disabled and can’t customer face, and there aren’t any adjustments that can be made for her, you need medical evidence AND documentary proof that you have attempted to find suitable adjustments with her to try and avoid liability. Explore as many alternatives as possible - offer breaks spread out across the day, reduced working hours, or alternate hours to less busy periods for example - and document all discussions and why the RAs are found not to be suitable.
But a lawyer/HR advisor would be your best solution I think!