They definitely have a pair available in FAM 210 as of writing this post - try the following:
1. If he did not 'add another member' prior to attempting the ST purchase then he needs to do this before he can have two ST s in his basket. Try to add two after adding the aother member if he didn't do so already - this may work but as I don't hold two memberships without STs already purchased so I haven't tested it.
2. To get two STs into his basket at once then add them 1 at once without logging in at all initially - to do this add the first ticket then click the back button on the web browser and then add the second ST at the other price band. This does work and I had two STs (1 adult 1 junior) in 'My basket' and they were next to each other. If he then clicks the checkout button it will prompt him to log in and then add the second membership number.
Hopefully that should work!!
3. Failing that ring up ASAP tomorrow as there will be very few pairs of tickets left - it may also be worth asking for 1 behind another if a pair is not available. Failing that then 2 very close together (especially if seperated by 2 seats) and then ask the guys in those seats to budge up.
Good luck!!!