For two lines, a dual sim phone is much easier. The problem is, in the US, these types of phones are hard to come by, and have some issues. For instance, google's cell service is one where you can get and easily use a dual sim phone, but your second line will not be "captive" so certain carriers will not allow certain things. If you use a phone captive to a major, to get all the services, then they likely prevent use of the second sim, even if the phone has it.
In my case, the problem was the second line would not use wifi when cell service was not available. I need a major for best connectivity, and the other major for times when the other has no service (when traveling). Two phones (actually three) is what I end up with to accomplish what I want. One for each major, and a google cell as a normally "suspended" alternative. Creative call forwarding settings means whatever phone I have on with reception reaches me. (I also carry a SpotX satellite handheld, so I can text message out when there is no cell coverage.)
On the OP, it always makes me smile when such a small change to taxable income causes so much kerfuffle. Most TP's who ask are not in a high enough bracket where the extra taxable covers the time spent researching. Employer can make it easy by a taxable addition, say for the desired amount plus 10%. As someone who has two family members who will eventually depend for a large part of their ongoing income based on my SS numbers, anything which can maximize taxable pay up to the SS limit is not a bad thing.