Assault Tiers play differently from Territory Tiers. Instead of dividing the Defenders between two Objectives, the Defenders quite often are more interested in the lone, mobile threat - the Bomb. Once they find it, they camp it until it resets.
Breakpoint has Dual Territories. They are orientated in such a way that Breakpoint can play Tier 2 as a Territory Tier with no modifications to the map (the Bomb spawn is simply not used). Other aspects of the Tier 2 design lend to both Assault and Territory, such as having two walls blown off when the Bomb explodes, or opened when a Territory is captured. Obviously, Assault is more realistic from the aesthetic point of view.
Unlike Boneyard and Spire, Breakpoint's Tier 2 Defender Spawns are off centered, from between the Objectives, to way off to the Bravo side in the garage. This design is more appropriate to an Assault Tier since the Defenders' focus is not so much the Objectives.
Instead of a Defender jumping into a Territory to stop the Territory timer from counting down to zero, the Defender must jump onto the Bomb Goal to disarm the Bomb before the fuse gets so small that there no longer remains enough time to disarm the Bomb.
Instead of an Invader staying in the Territory to capture it, once the Bomb is armed, the Invader can take up a power position to protect the armed Bomb.
In general, Tier 2 is a better choice for Assault Tiers, because vehicles are introduced with Tier 2. The presence of vehicles distracts attention from Objectives; and vehicles also support camping of the Bomb anywhere on the map.
A very real concern for Assault Tier forging is that the Bomb Spawn must be heavily isolated or protected from the Defenders, lest the Defenders simply camp the Bomb Spawn and never let the Invaders have access to the Bomb. Breakpoint uses distance from Defender spawns, discrete channels of paths making their approach predictable, and a structure that the Invaders can control (as opposed to in the open where their vehicles spawn).
Finally, there are several bugs in the Game Type that you need to consider before you forge an Assault Tier into your Invasion map. The Fuse Time and Arm Time are identical - they cannot be set to different lengths of time. The sudden death feature is broken on the Assault Tier. And the bomb does not reset if it enters a Kill Boundary.
While the first two are workable, though problematic, the third bug is a deal breaker for most maps. A map with paths that come anywhere near a Kill Boundary (e.g., the ocean) has the potential to stall a Tier indefinitely with the guarantee that the Defenders will lose once the Tier timer expires. (Yes, Breakpoint suffers from this bug.)
Summary
Though you can have Dual Objectives for Assault, only one Objective can come under attack, and there is only one mobile threat - the Bomb.
Assault is best forged into Tier 2, where vehicles and power weapons are already taking Defenders away from guarding the two Objectives.
The bomb spawn must be protected or isolated from the Defenders, lest they camp the bomb spawn to prevent the bomb from ever reaching an Objective.
There are several bugs with the Assault Tier, the most significant being a potential stall of the Tier should the bomb be dropped into an inaccessible location, like the ocean or a gorge.