I don't think you will gain anything at all flying 2 sails at the same time. As Faster posted, you will have to back up the deck attachment with an attachment to a bulkhead or the hull belowdecks and runners will also probably be required. Trimming a cutter rig is not as easy as trimming a sloop rig either.
Often a Solent stay is installed inside of the forestay, it is removable and used for storm sails but not with the other jib.
What 2 forestays do give you is more flexibility. As an example the Saga 35 & 43 by Bob Perry have a "variable geometry rig". This allows the larger sail forward to be used downwind and in light air and the smaller inner sail to be used in stronger winds and upwind. They are both on furlers.
Living aboard in Victoria Harbour