OK to use Splitter for 2 Subs-Receiver has 1 port

Hello Everyone,

Is it OK to use a splitter with 1 male into Onkyo 905 single pre-out in order to use 2 subs (Velodyne DD-18 & currently unused M&K sub)?

I called Onkyo and they said it could potentially cause damage to the Onkyo?

Thanks in advance for your input,

Dave