It's not logical, because the poison is consider like a spell for the critic, but the AP make it increase (not SP)?
Do you know what i mean? I'm sorry for my bad english

Simsheet does not assume anything about DP dropping or not. Its assumptions only involve when you swap back the dagger with DP poison, and that's something you decide on. Let's say you choose to swap it back with 3 seconds remaining, then the simsheet will go on with a swing reset on both hands and regular RNG rolls that determine if your offhand attacks land, DP hits and then applies. If that happens before DP runs out, it is refreshed and 1 second later (reaction time) you put IP dagger back on. So it's a complete simulation of what happens in reality. Also as a minor comment, if DP is refreshed after 4 ticks of DP has already happened, next DP tick is reset.

So if you are not letting DP drop artificially by shiving or something you are preventing those two costs: the cost of restacking DP and the cost of potential tick reset. However, you are incurring the cost of having to use a shiv, which is highly inefficient use of energy. In addition, you are also making your life harder because you are timing your shiv. It's much easier and probably better in terms of dps as well, to just set the swap back time that maximizes youroverall dps. For instance 3 seconds usually gives you about 15% chance DP will drop if you did not envenom right before it and 5% chance DP will drop if envenom buff is active during the swap. But that's fine since overall that seems to be close to optimal swapping back time given all costs and benefits involved.

As for 2-piece bonus, it's built into the simulation as well, so it can measure the effect of DP dropping and DP tick resets on the effectiveness of 2-piece bonus for you. So if the simsheet tells you having the two-piece bonus is slightly better than the two ilvl245 pieces, I'd go with that and would not even try to do anything special to keep DP up more, just follow the rotation suggested by the spreadsheet.

It is a simulation and as such there is a great deal of randomness in individual trials, for example the occurrence of crits, and the natural range of white damage and finishing moves, relentless attacks, lethality, and berserking procs.
The accuracy from this sheet comes from doing a large number of simulations and averaging the results.

This takes time and makes this sheet less useful for making quick gear decisions and arguably more accurate for finding a single best in slot set, as it accounts for things like trinket synergy, the effects of fight length on sustained dps, and other things that cannot be shown in a spreadsheet which is modeled mathematically.

I pulled out some numbers that can help you decide. I am constantly updating my cheatsheet for choosing swap out offhands, but it's not in a shape yet to make it usable for everyone. Here are some numbers from it though:
1. The cost of 0.1 speed for IP dagger is 38 dps
2. The cost of 0.1 speed for DP dagger is 150 dps
3. The uptime of IP dagger is 76%, the uptime of the DP dagger is 24%

So first thing is that you definitely want a fast dagger as DP offhand. Second, by using Twin Spike instead of Steel Breaker in main hand you lose only about 40 dps in BiS gear. At the same time using the DotNW in main hand is about 200 dps loss. Finally, using regular Dirk over heroic Steelbreaker as DP offhand is about 40 dps loss.

All in all, i'd say two choices seem about equal:
- MH Steel, IP OH Steel, DP OH DotNW
- MH Twin Spike, IP OH Steel, DP OH Steel

I'd personally go with the second one because I don't know about you, but I am using the Outfitter script to swap and I am not so sure it reliably swaps my offhands at 3 seconds remaining, which would ensure low uptime of the DP dagger. For me I set the Outfitter to 5 seconds and because of lag it ends up swapping some timebetween 2 and 4 seconds. If you set it to a higher swap time, the cost of using ilvl232 DP dagger instead of an ilvl258 dagger will go up.