There aren't combined bus and rail singe tickets, as I posted you can buy the i-link card online but only if there is enough time to have it posted so if it's this Thursday then no.
Otherwise the only option is two singles, bus in cash to the driver, rail at the station ticket office.
There is one alternative, though it may leave you temporarily £1.50 out of pocket compared to separate single tickets. Much more likely it'll save you £4.50 though.
Buy a £9.50 Bus Rambler ticket on the bus, and ask to use this on the train under the Ticket Transfer policy.
The Ticket Transfer policy states that Sunday Bus Rambler tickets can be used on the train after 6:30pm. The Bus Rambler ticket that's available daily over the summer is a promotional extension of the Sunday Bus Rambler, and Translink have previously confirmed on Twitter that the transfer applies daily, not just on Sunday.
https://mobile.twitter.com/Translink_NI/status/1075064445309739008
(They have, confusingly, also confirmed on Twitter that the transfer doesn't apply daily, and is restricted to Sundays, and it's not clear which is the actual intention. So if the Bus Rambler is refused on the train, buy a rail single and then chase Translink for the extra £1.50 paid)