Well at least you'd be starting with a big enough dust collector...
The short answer is it's not going to be 100% safe. And although I can think of a way to hook it up being mostly safe, the expense would probably exceed the cost of buying another dust collector just for metal. You would have to design a system that would eliminate any possibility of cross contamination between the wood and metal dust while using the same fan on both systems. The fan would have to come after the cyclones and filters, which would mean some serious plenum building and some big blast gates.
Even with dedicated dust collectors just for metal, the manufacturers reference "flame events" as a possibility. When I put dust collection on the grinders in the metal working area, I used both spark arrestors and cyclones before the fans, and had flame retardant MERV 15 cartridge filters custom made. A spark would have to make it through the arrestor and the cyclone to start a fire in the collection bin, or go through the fan and into the filter plenum, which would ruin the filters, but not burn down the shop.
Air traveling in a dust collection duct is typically traveling under laminar flow conditions, a spark in the duct will be traveling at the same speed as the air around it and quickly becomes surrounded by warm oxygen depleted air. The spark stays hot, and does not "burn out" because it is isolated from oxygen. Spark arrestors stop sparks by disrupting the laminar flow and slowing the air. As the air mixes around the spark, oxygen gets to the spark and consumes it, and cool air cools the ember.
Spark arrestors are surprisingly expensive and many have lead times that imply they are made to order. I went with Nordfab since the units were available at a somewhat reasonable price and lead time. Spark arrestors differ in how they disrupt the airflow, and the manufacturers will recommend specific lengths of duct before and after the unit, 10 duct diameters being pretty typical. So for the 6" duct I used, 60" or 5' on each side. In my case I wanted the dust collector a bit closer than that to the grinders so I bought a viewing spool and did some testing. I found the 10 duct diameters on the outlet was critical to eliminate 99.9% of the sparks, but the length on the inlet side could be reduced a bit and the units still worked. YMMV with different brands, I have a different spark arrestor from another project, and a lot of sparks get through unless there is at least a 10 duct diameters length of duct on both the inlet and the outlet. A cyclone can also be a spark arrestor, (for smaller systems the steel Dust Deputy surprised me with it's effectiveness) but given the design of the dust collectors I bought, the only way to test the cyclone would have put the filters in jeopardy, and each spark arrestor cost only slightly more than a new set of filters would have...
BTW the dust collection cabinets with the 4" inlet like the Jet don't move enough air to be worth buying for a grinder. I would suggest another 5 Hp dust collector if you want to use both grinders at the same time, 2 or 3 Hp if you would only be using one machine at a time
