If both of these batches are the same size, the combination would give you a gravity of 1.120 with 7.5% ABV and if the EC-1118 is able to chew through it all, you should be able to get the gravity down to about 1.040 with 18% ABV. What sizes are these batches?
First question is do you have a container big enough to hold it all? If not, you may need two carboys to work with. I'm going to address this as if they are both 1 gallon batches.
You wouldn't want to mix it all up at once as that might stress the yeast quite a bit. It would be better to get the new batch started; I'd rehydrated with GoFerm, and pitch into the new batch. I'd aerate it well, and let it get rolling, with a double size dose of nutrient at the end of lag phase and I'd keep aerating the snot out of it. Once it was going strong, with a gravity down to about 1.070 or so, I'd add the old batch gradually to it 1 quart at a time (If I didn't have a container that would hold it all, I'd now split it in to two batches and add a pint to each).
After adding each quart, I'd measure the gravity and aerate. In theory, after the first quart, the gravity should rise to about 1.080, and the ABV will have gone from 5% to 7% which the yeast should be able to tolerate. Each subsequent addition will have a reduced impact. When the gravity drops back to 1.070, I'd add the next quart. Hopefully, it will all be added over a period of about 4 days and with a little luck it will ferment on down to near the ABV tolerance.
One additional step you can take to make the old must more fermentable is to pasteurize it. I'm not sure why this helps, but it is well documented in wine literature. I'm not a big fan of heating mead musts, but I might consider it in a situation like this to make sure that it gets finished.
Alternatively, you can just let the new batch go dry, and blend to taste, and if it referments just keep adding the old must to taste until it stops. Doing it this way may still leave you with some left-over sweet must, but you can soak some fruit in it and pour in on vanilla ice-cream - yum!
Please let us know what you decide to do, and how it turns out.
Medsen