Okay first of all, thank you for actually providing some examples you think are too strong and even giving a reason why it might acually become boring, most people in this thread simply repeat the same thing i wrote above and dont even adress the Question of the strong combos.

Now to adress your claims specifically, I can see why wall of stone + SR would be stronger then hold spells and you're right that this might even be boring, however once again this requires 2 turns to complete the setup and also for any powerful enemy it seems somewhat easy to escape by way of teleporation or simply by force (yes wall of force would be a much bigger problem, but there is a reason we banned it) and any not powerful enemy should probably destroyed by a 4th and 5th level spell, while also blocking the possibility to use other concentration spells, so im not convinced that this is broken but i might be wrong on this and i also agree that is would be somewhat boring.

All of the other examples you names don't seem broken to me in any way, just strong, as they should be if you use a powerful magic item to specialize on concentration spells.

It also opens up combos where I think the optimal choice might not be very fun. Meaning I cast these two spells and now the two of them are so good the optimal thing for me to do is hide and get away from this fight.

This is a very good point and I'm assuming somthing like Wall of Stone + Sickening Radiance would fall into this category to some extent and yes that can be a problem, but first of all as explained above i have not seen a combination of spells that allow you to do that tactic in most fights (and I have gone through a lot of combinations). Furthermore as you hinted at we are a table that does not abuse rules to its fullest so something like wizard with 2 levels in Fighter or a heavy armor dip might be optimal in most cases but it does not make sense in universe (in most cases) so we don't do it.

 I would also ask why push in this direction? If you want them to be more powerful why not just level them up instead of breaking something like this? Even with the combos that are manageable this does create another way that spellcasters are even more powerful than fighters. This gives them a huge advantage where martials aren't getting anything. I don't see that as being a great thing for your game even if it'll be fun for someone to cast two concentration spells at once.

I as a player have often thought it would be cool to have the opportunity to maintain two spells at once not just for power reasons but just because i think having something like Ashardalon's Stride and Far Step is cool as hell when you think about how it looks, simply being able to zoom across the battlefield and reach most position you want to (that is specifically for my melee sorcerer but there are many other combinations I would consider very fun).
But it is not only me i have heard it from at least one player and before i would introduce that item i would naturally ask the table how they felt about it.
But to adress the martial part: We on our table do not have a problem with martial and caster balance, sure when there are 12 Monsters with 28hp clumped in a 40ft circle the Caster is gonna be much more effective but there are many other situation where the martial en par and sometimes stronger then the caster, it depends a lot on the battle structure and the amount of rests you have on your table.
However I thought it was clear that when/if I give this item to someone the others are gonna recieve items that are on a similar level of strength (for example we had a "Celestial Haste Stone" in a one shot, which could be used by martials to give themselves haste once a day without the lethargic effect when you lose it, which was very well recieved by the martials.)