I am modifying my existing deck. I have plenty of 2x8x6 PT boards. I want the new span to be 18ft. Can I double up a 2×8 and place joist hangers on both sides, and place my beam directly under the the splice area. The 2x8x6 will be connected to the house ledger board and tie into the splice board, with 2x8x12’s attached from splice and supported by another beam at the edge of the deck. Any thoughts or suggestions.
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18 ft? Now you're talking 'house size' dimensions. I suggest you look under your house, then duplicate what you see there. If they used 2x12's, you use 2x12's- no matter how many 2x8's you'd love to use instead. If they have supports every six feet, you do the same. If their posts rest on concrete bases, you do the same.
In other words ... copy someone who knew what they were doing.
I believe that 12 feet is about the max span for 2x8 in regular construction. So long as you don't have a serious snow load to worry about it would seem to be OK for decks too, but you should check some span tables to be sure.
It's perfectly legit to split a long span into two shorter spans, with a beam in-between, either with the joists fit to both sides of the beam with hangers, or with the joists sitting next to each other on the top of the beam.
If you have a carrying timber/beam in the middle +- then you don't have an 18' span, you have two spans corresponding to where the beam is placed. 12' is the maximum span for 2x8 16" OC. There will be a fair amount of deflection, a bit too much for a house interior, for me. Going to 12" OC will help a lot with that and only require a few more joists. You don't have to double up if you are splitting the span.