A speedball is usually Heroin and Crack or Coke where i'm from, but there's nothing stopping you creating your own personal pseudospeedball. I used to always mix opium and mephedrone, or Meph/MDMA and GBL (wonderful combination). Even Kratom and Mephedrone is wonderful.
It usually specifically refers IV'ing H and Coke, but essentially refers to a stimulant and downer combo. It's just a term at the end of the day, it's up to you what you want to do and how you go about labelling it, but the important part is staying safe. It can be fairly dangerous if excessively abused and without proper care.
I suppose meth and opium could be called a speedball, it's just a term at the end of the day. It would be like calling a Mojito a Mojito but using vodka instead of rum. I mean, it's essentially the same thing, but not quite.
I am not one for IVing either...I would usually drop 500mg of Mephedrone into a shot of vodka which I would add 2ml of GBL to. Do this several times a night with plenty of Meph lines, joints and plenty of drink and you're golden. However, this is terrible advice, given it is such a dangerous combo. It's lucky I survived some of the combinations and amounts I used to do....I should have died on several occassions, truly.
I'm veering off topic though...I would say just go for lowering your dosages of each by half and ingesting them however you wish. If it was me personally, I would ingest both of them intra-rectally or orally in the same vehicle.