Any advice on putting together a cake with one buttercream tier and one fondant tier?

ABe careful? I dunno, I just do it...lol. Maybe chill your bc one before assembling it so you don't mar the icing?

I wondered if there is any issue with doing a buttercream tier on top of a fondant tier...

As long as the top tier is well supported, this shouldn't be an issue as the weight will be on the support, not the cake.  Shouldn't be any different than stacking two buttercream cakes.  I agree with AZ to chill so your are less likely to end up with fingerprints in the buttercream.  This might seem obvious, but I would add the border after you've stacked and, if the design allows for it, plan to have an extra few decorations to cover up any mistakes.

This cake has me confused, I swear!  The lady wants a picture of herself on the top tier...and the only other request is that I would use the colour aquamarine as that is her birthstone.  I have no idea what to do!  I've never done a two tier with an edible image on the top!  I am used to making flowers, etc. to go on the top of a second tier!

Sounds like you can do just about anything to go along with the edible image!  Is that why your are thinking you need to do that tier buttercream, or is the buttercream part of your design?  If you aren't already aware, you can attach edible images to fondant.  Does she want it on the very top, or can it be on the side of the top tier?  I know it's been done to death, but I'm still in love with ombre ruffles.  You could do aquamarine ombre ruffles (maybe ruffle rosettes?) on the bottom tier, and put her edible image in a fondant or piped antique style "frame" as if it were a cameo pin on the side of the top.  Finish with a ribbon and some light orangey pink flowers (or white if you prefer).  Sigh.  Can you tell I am tired of making rainbow cakes and mermaids?

Could you do a sugar picture frame for the pic and prop it on top of the cake?

Thanks for the ideas!  I have just figured out that I am more a cake decorator than a cake designer.  This is a skill that I need to work on!  I love direction!  haha!  Well, I don't have much more time to figure it out, so I'd better get on it!  I really appreciate your responses, though.  Sometimes I feel alone, as none of my friends or family make or decorate, and so it is hard to find ideas that are doable for me!

No problem!  For me I'm usually dumbfounded when they don't want to have more say on the cake.  Not from a design perspective, but from a "I'm a control freak and I could never let someone just surprise me" perspective.  :D  Good luck with the cake!

Thanks for your reply, MBalaska!  So, I ended up putting fondant decorations over the buttercream, which helped.  I also have realized that I am a cake decorator, and not a cake designer...yet!  This was my first cake that was fully designed by me, with minimal input.  It stressed. me. out!  No joke!  Anyhow, it was done, she loved it, and it is now behind me! :o)  Thanks for the encouragement, everyone!