I had the 7410xsv and a 18HD radar on the Venture 34 in Costa Rica and it worked, but radar on split screen is not very useful as too small window.
The cheapest solution would be to find a 7608 still in stock somewhere and use it only for Radar, but you will get no updates anymore. I had also a 7408 on my RIB and could run eco and charts together, but obviously it was small and when i added a 8410 it became much more useful. More recent and faster would be a 8607, 8608 or another 8610.
The GPSMAP 8nnn series shines in particular due their HD screen, much higher resolution than the more economic 1243nn and 943 series, but the higher resolution screen of the 84/87nn series comes at a considerable higher cost.
The 9nn and 1nnn series are single channel xsv devices, so you can't show same screen low and high chirp or twin fixed frequency as it's possible on the 7/8/9nnn series, only either low or high chirp or fixed frequency. This may matters a lot, or nor really.
You can save about 200$ buying the non xsv version and network them to still have radar, echo and charts on both mfd's and can use whatever function on each of them.
The 7xxx and the 8xxx series have two network ports so you don't need a network switch, just plug the radar in one and add a second network cable to connect both mfd's together and you are good to go.
Consider that it's more difficult to sell a non xsv version than a xsv. I am stuck with this problem to sell my 8410.
All depends on how much you want to fork out and how much space you have available to install a second MFD.
On the 306 Canyon i bought a few months ago i have two 8412xsv and thats better than 2 x 8410xsv as my seat position is much more far from them than the 7408 and the 8410 on the RIB i had before.
If the distance of your mfd's is rather near then 8 or 10 inch will work, if distance if medium then it becomes more difficult to read them.
Keep in mind that you get used to bigger screens ... I start to think about to keep my almost new 8410 and add it to my two 8412xsv, either on top of the console to show radar and/or nightvision or hang it under the rear of the hardtop to see the depth sounder when i am fishing from the transom.
Chris