GP2 and Formula E racer Daniel Abt and ex-Formula 1 test driver Rodolfo Gonzalez are both exploring IndyCar moves.
Gonzalez ran with Dale Coyne Racing earlier this week, while Abt is scheduled to test with Andretti Autosport for the first time next month.
The 21-year-old German is slated to spend a day in the team's Dallara-Honda at Barber Motorsports Park on October 27.
Abt, who was GP3 Series runner-up to Mitch Evans in 2012, made his first visit to an IndyCar paddock at last month's season-finale at Fontana, and admitted at the time that a move to the US could be on the horizon.
"It's an option in my mind, but before I can really make a decision I need to get a feeling for the sport," he said.
"Formula 1 was always my dream and that's why I always did GP3, GP2, Formula 3, whatever, but it's [Max] Verstappen at 16 taking those places [in F1 teams] and I'm starting to think that maybe it's not what I want to do.
"You can always bring some money and drive a Caterham, but is it worth it just to be called an 'F1 driver'? I don't think so.
"I'm 21 now and I'm at a point where I need to look at what makes sense."
Abt was a team-mate to 2014 IndyCar rookie of the year Carlos Munoz at Signature during the 2011 Formula 3 Euro Series season.
Gonzalez (pictured right), who was Marussia's F1 reserve last year, drove for Coyne at Barber on Monday this week.
The 28-year-old Venezuelan was a long-time stalwart of European junior categories, although his four seasons in GP2 never produced a final championship position inside the top 20.
He made his Le Mans 24 Hours debut this year in a Murphy Prototypes ORECA-Nissan LMP2 car alongside Nathanael Berthon and Karun Chandhok, and is also contesting the European Le Mans Series with the squad.