It is possible to have both Dagger and Dagger 2.0 installed at the same time, in separate Incognita program slots.
Dagger and Dagger 2.0 break firewalls for PWR with no side-effects. They are PWR-efficient breakers; Dagger costs 0.33 PWR per firewall hacked, and Dagger 2.0 costs 0.6 PWR per firewall hacked. Their cooldowns are too long to use them as your main breaker, but they can make a good addition to your loadout.
If you have sufficient PWR generation, then other secondary breakers may save more PWR overall, even if their PWR cost per firewall hacked is higher. For example, Lockpick costs 6 PWR to break 3 firewalls; compared to this, Dagger saves at most 5 PWR every 5 turns, while Wrench 3 saves 3 PWR every time you hack a device with exactly 3 firewalls, which may be more frequently.
Dagger or Dagger 2.0 can work well with Parasite in a low-PWR-generation strategy. Since they break firewalls instantly, you can use them to hack Security Cameras earlier to explore the facility sooner. However, later in the mission, it is often best to keep them off cooldown in case you need to hack something immediately in an emergency — especially drones in Sankaku missions.