Covering silver wire for use as a speaker cable

Here's a basic "rule of thumb" for estimating wire gauge needed when combining gauges. There's a chart for this somewhere.
Every time you double up the amount of silver, the effective wire gauge goes DOWN 3 steps.

If you double-up on 18 ga., the effective total gauge is now 15. Double up again (4 - 18 ga. wires), and it is now 12 ga. effective total.

Add two eight ga. wires together, and it becomes 5 ga. effective total.

So in your case, doubling two 26-gauge wires is now 23 guage. Four runs of 26 guage would now be 20 gauge, and so forth...