I have had this in the past. I've ended up just going through the procedure to buy one ticket twice.
My gripe with the site is that it insists I should register as a customer with them before proceeding to buy. I complained to GWR▸ and was told that they were aware of this and looking into it. I am afraid I just resent having to register with any vendor for casual purchases, and it's worse when it is what seems to be effectively a monopoly supplier. Even Amazon (that I have reservations about using having regard to their adverse effect on other retail businesses) seem to have no problem allowing you to "sign in as a guest".
The change of the underlying web site / engine a few months ago has certainly removed a number of facilities that were useful, or made them harder to find. A move from a useful site to one that's functional for straightforward use.