That ultimately wouldn't work either due to resources being eaten up
If we do get dual protags, the Inquisitor would most likely have no companions with them when playable
Why? We had 9 Companions and 3 High Dialogue Advisers in DA:I.
Even if the Inquisitor only got a team of 3, then with the 6 designated for the New PC we would end up with a grand total of 9 Companions. 9 Companions in One Group in DA:I meant there had to be an enormous amount of dialogue between them to facilitate for every group combination (and if you look at the wiki there is an absurd amount of this Party Banter). 9 Companions split between 2 groups would significantly decrease the amount of group combinations and even if those companions bantered with each-other a little more, Bioware would still save money because they wouldn't need to compensate for so much variety.
Even story missions would be cheaper. Can you imagine how much it cost to record "In Hushed Whispers" with 7 Companions on the optional party list (Solas, Varric, Cassandra, Sera, Blackwall, Iron Bull, Vivienne)? If each party had fewer companions and each PC dealt with their own stories for much of the early game (only coming together near the end), that would mean Bioware would have to record far less "optional" dialogue for these missions; because there are simply far less options for companions to be in them. The Inquisitor has a party of 3 that doesn't change, they always record those three guys for that story mission dialogue. The New PC has a group of 6, but still a party size of 3 Companions ... which means at most they would have to record 6 different options. Still saves money on "each" story mission in the long run.