Hi,
I think the intended upgrade was replacing WLE900VX with new Wi-Fi 6 card.
Then use 2 antennas with diplexers to 2,4G + 5G and 2 antennas for 5G only.
So your option (2).
If you would like to use your option (1), you can buy one more antenna.
For ex. I plan to go with upgrade kit, which will use to replace WLE900VX. Then I bought AR5BXB112 card: https://www.aliexpress.com/item/836990135.html?spm=a2g0o.order_detail.order_detail_item.4.278ef19ctqenCt
which is 3x3 MIMO for 2,4G so I will replace also WLE200N2 card.
Then I bought 1 more antenna with 1 more RSMA pigtail.
So I will have 2 antennas with diplexers, 1 antenna 2,4G only and 2 antennas 5G only.
The reason for this are common problems with clients connection and I found here on the forum that someone tested this Atheros AR5BXB112 card with Omnia and it works good and the problems went away. Hope it will works 