TL;DR; This can be easily fixed by saying Hi user1 and user2! Proper usage of the To:, CC:, and BCC:, can also help signal whose attention you are trying to get. Whether you say good morning user1 and user2 or hi is up to you but there is nothing wrong in putting both their names there.
That is a bit of a hard one to answer without understanding the relationship of the group. Is this school or work? I assume a workplace but if that is the case, are they co-workers? Management? Subordinates? I always try to choose wordings that best target my audience. To whom it may concern is a popular choice but comes off as a bit distant. To me, that is typically used when sending a letter to a business and not sure who would be the one receiving it. Good morning, and so on as Thomo said in his comment is usually one of the best ways to be polite and professional and one of my favorites to use.
I am slightly thrown off though because you are looking for feedback from 2 specific people in hopes 1 would reply. Not sure how a change of greeting would signal someone you want them to reply. The best most direct way for this usually found in e-mails is directly asking for it in the body. Or you can simply say Hi bob and jenny!
Also, how you provide the grouping of recipients helps out a lot too in determining feedback. Are the 2 users you want feedback from both in the To: section? Users in this section know that this email is directed TO them and not simply added for FYI. in the CC: section people here usually know they are added in for the FYI with the ability to chip in should they feel the need to. the BCC: is where you add people like management or uppers who you want to see the email and everyone who was attached so that they can stay informed without possibly freaking out the main audience that management is also in on the email and they can respond with honesty instead of what they think management wants them to say.
