Fighter 5/barbarian 2

One attack with Advantage is worse than two without, unless you have Sneak Attack*.

Damage comparison

In both cases, you roll 2d20:

• If neither is high enough to hit, the result is the same
• If only one is high enough, the result is still the same
• If both would hit, 2 attacks do twice as much damage

As you can see, Advantage is identical in some cases, clearly worse in others, but never better.

Reckless Attack is not free

Providing Advantage yourself is quite a steep price

Other sources of Advantage exist

If your enemy is prone, restrained or provides Advantage for any other reason, your two attacks just got that much better. In the other case you DPR does not improve, you just don't provide Advantage.

tl;dr: Even if a 2nd level barbarian always had Advantage and without a cost, Fighter 5 would still have better DPR than Fighter 3/Barbarian 2

*If one of your allies is standing next to the target, two attacks are better than Advantage on one attack, even if you have Sneak Attack

Fighter 5/barbarian 2

One attack with Advantage is worse than two without, unless you have Sneak Attack*.

Damage comparison

In both cases, you roll 2d20:

• If neither is high enough to hit, the result is the same
• If only one is high enough, the result is still the same
• If both would hit, 2 attacks do twice as much damage

As you can see, Advantage is identical in some cases, clearly worse in others, but never better.

Reckless Attack is not free

Providing Advantage yourself is quite a steep price

Other sources of Advantage exist

If your enemy is prone, restrained or provides Advantage for any other reason, your two attacks just got that much better. In the other case you DPR does not improve, you just don't provide Advantage.

tl;dr: Even if a 2nd level barbarian always had Advantage and without a cost, Fighter 5 would still have better DPR than Fighter 3/Barbarian 2

*If one of your allies is standing next to the target, two attacks are better than Advantage on one, even if you have Sneak Attack

Fighter 5/barbarian 2

One attack with Advantage is worse than two without, unless you are a Rogue.

Damage comparison

In both cases, you roll 2d20:

• If neither is high enough to hit, the result is the same
• If only one is high enough, the result is still the same
• If both would hit, 2 attacks do twice as much damage

As you can see, Advantage is identical in some cases, clearly worse in others, but never better.

Reckless Attack is not free

Providing Advantage yourself is quite a steep price

Other sources of Advantage exist

If your enemy is prone, restrained or provides Advantage for any other reason, your two attacks just got that much better. In the other case you DPR does not improve, you just don't provide Advantage.

tl;dr: Even if a 2nd level barbarian always had Advantage and without a cost, Fighter 5 would still have better DPR than Fighter 3/Barbarian 2

Fighter 5/barbarian 2

One attack with Advantage is worse than two without, unless you have Sneak Attack*.

Damage comparison

In both cases, you roll 2d20:

• If neither is high enough to hit, the result is the same
• If only one is high enough, the result is still the same
• If both would hit, 2 attacks do twice as much damage

As you can see, Advantage is identical in some cases, clearly worse in others, but never better.

Reckless Attack is not free

Providing Advantage yourself is quite a steep price

Other sources of Advantage exist

If your enemy is prone, restrained or provides Advantage for any other reason, your two attacks just got that much better. In the other case you DPR does not improve, you just don't provide Advantage.

tl;dr: Even if a 2nd level barbarian always had Advantage and without a cost, Fighter 5 would still have better DPR than Fighter 3/Barbarian 2

*If one of your allies is standing next to the target, two attacks are better than Advantage on one attack, even if you have Sneak Attack