You will struggle to light a wide angle lens such as the Tokina with a a DS51. You would also struggle to light it with a single DS160 for many shots BTW.
There is a minor colour temperature difference between the DS51 and DS125/160 but in practice I have used them together many times without noticing.
For a few years I used a DS51 as a backup strobe and suffice to say it got a lot of use.
As previous poster pointed out the other issue is weight - they do weigh a lot and with airlines becoming more antsy re weights that also killed them for me.
Are two better than one - difficult call - some shots are simply better with one, often macro where you can light from a side or angle and add depth to a shot that will not come about with the often more flat light rendering you get with two strobes.
I might pick with the previous post a bit in that both have (or can have) rechargeable battery and the DS51 proved far more robust than any DS125/DS160 that I owned.
Paul C