Question: So i subbed L to my Fortress main and they share the Iron Wall skill, do the effects stack though? Should i invest in it as F AND as L to get the most out of it?
W-why would you sub Landshark to your Fortress? Just for Iron Wall? Not worth it even if they do stack.
As for whether or not the stack, I can't confirm on that skill, but if I recall from what I've read, skill stacking in EO4 does work but the subclass' stacked (same) skill has a reduced effect. Basically it's not worth it.
Would highly recommend you consider a better support class for your F's sub. If you're building your F for some damage, their innate damage skills are actually pretty good, so just invest in those, because you will not get good damage out of half-rank L skills on your F. If you're building your F for tanking, then you want subclass options that help them support the party better. Almost anything is a better pick than L, no matter what your party build is.
As subs for F your options are... (my recommendations in bold)
L - Iron Wall, but as mentioned you have better options.
N - Speed Boost is not bad but that's about all you're getting.
S - Support skills like Spotter, Eagle Eye, Scavenger, or Discerning Eye, but...
M - Actually a decent pick. Patch Up stacks IIRC, and Stretch will help your Fortress avoid binds which is critical for them. TP boost can help for boss fights and the mace skills are good. Don't recommend trying to spend points on healing skills unless you're not going for any mace skills.
R - TP boost, Runic Shield, and Free Energy are all useful for F. Rune skills can be useful, too, if you don't have a R or /R in your party with those skills yet.
D - An excellent choice (IMO the best choice). Fan Dance is very useful for F as it gives a better evasion rate than Speed Boost. They can also afford to take Sword and Mist dance giving them chances to land stun hits on turns where they don't need to use a skill. Refresh Waltz and Freedom Waltz are literally life-savers later in the game and even if you have a dancer in your party already, letting F do these dances while D does something better (Attack Tango and/or a Samba). Quick Step is priceless and Burst Saver is a good investment for 3 points.
A - Warding Mist is very good, Tame Ground is good but not as useful in EO4 as in other games, Bracing Walk is good for saving on TP/items, and Eye skills are good picks for turns when your F can do something offensive.
B - Roar and Endure are good. Ferocity and Power Boost aren't bad to dump points in after important things are done. You don't want to be using Blood Surge as a tank. If you're trying to build a tank that can do damage, this is a way better subclass choice than L, at least IMO.
I - Hawkeye is about the only useful thing and it's only useful if you need it (and if you can afford the debuff slot) and don't have an imperial in your party already. Not a good option.