I don't think the AF bus that stops at Porte Maillot goes into Paris proper. Sure, that stop is really Paris, but it's on the far edge and I think the bus gets there from the airport by going around the peripherique, not through central Paris. That is where it enters Paris and then goes to l'Etoile.

There are a couple shuttle companies that advertise this, but it won't be cheap, that airport is very far away (almost 60 miles from Paris)--if you google you can find them (like Royal). That would probably cost around 100 euro. I've never heard of a bus between them.

There is actually a train station at Beauvais and then you could take a taxi to the airport from it, that woudn't be too far. The train leaves from Gare du Nord in Paris and there are direct TER trains about every hour. The trip takes about 1:15 from Gare du Nord.

So you'd take the RER into Gare du Nord and then get a train to Beauvais. The fare is only 13.5 euro from Gare du Nord. There are trains at 12:01, 13:01, 14:01 and 16:05 on some weekdays, other days may differ or weekends, but there are plenty, you'd have to check the SNCF schedules. The airport estimates the taxi from the Beauvais station to the airport is around 10-15 euro.
http://www.aeroportbeauvais.com/acces.php

Of course you'd have to make time to make those transfers and buy tickets (for the train), as you cannot buy one online and print it out yourself.

Oh, sorry, when you said "between the two airports, I was thinking Orly and CDG. Forgot you were looking to get to Beauvais.

Appreciate all your input.
I found these shuttle services:
Always shuttle & Mondial shuttle. Both are 80 EU from door to door for 2 people. really not that much more than taking other transport.
This may be the way to go as the other options thus far sound like we'll be racing the clock.
It would be simpler to go to Venice instead as we wouldn't have to leave CDG but there aren't many places to stay in Sept.

That's not bad at all, I'd pay 40 euro a person for the convenience, if you go that route. Because even taking the train would cost you around 8 euro into Paris, then around 15 euro on the train and another 5 euro in taxi (per person), something like that.

Reporting back that I did end up taking the first flight that I originally asked about despite having reservations for a 2nd (additional) flight and the shuttle service. The shuttle service canceled on us last minute so we're contesting those fees.
We did, however, have to eat the cost of the 2nd flight.

We did not have to go through customs because we flew through Iceland.

We managed to get a passport stamp, get our Icelandair checked in luggage, go from terminal 1 to 2, check in luggage and pay for it, get to the EasyJet gate with time to spare.
Not that I'd recommend doing things this way but it did work.