What you need to remember is that for you to get the 2nd phone for free, you need to start a new line for it for $20 a month (which you already know) but you also would have to keep it on that line and active for 2 years. Do anything to change that (cancel the line, sell the phone, etc) and you lose the monthly bill credit that makes the phone free. Unless you really need 2 new phones AND you don't plan on upgrading for 2 years, I wouldn't do it.
When I went to order my Pixel 3, they were pushing me to do the BOGO. The rep I talked to feed me a big line of BS (that I didn't need to start a new line when it says right in print that you do, that I only had to have it active for a few minutes even though I'd lose all my bill credits and pay full price for it, and that I could sell the 2nd one off and get both phones for free). After getting burned a couple times by Verizon now, I will not have anything to do with any offer they have that includes monthly bill credits and locking me in for 2 years.