My daughter is coming up on 17 months and I noticed it becoming increasingly difficult to keep her attention. However, I've found that it really does depend on the topic. Some things she loves and will pay attention, other things she doesn't understand or simply doesn't care about, so she doesn't care to focus. Also, I find it more difficult to work with her if she's really wanting something else. So here's a few recommendations (just based off personal experiences). Please note that they aren't tips that all work together, but just some ideas of different options you could try.
1) establish a routine or sing a silly song just before starting, something that draws your child to you and lets them know that they're about to do something different/fun. Somewhat like that which many people do to help their child settle for bed- a routine before sleep makes a huge difference in how easily our daughter will go to bed, so I figured it might help with other things too. She actually really enjoys doing things that she "knows" about. For example, I try to get her to help pickup toys and it rarely works if I just try to interrupt her and tell her to put her toys away, but if I sing our "clean up song", she happily joins in. Same thing when I leave the house without her, the routine helps big time!
2) the idea posted above about putting bits in the hallway sounds great. I was going to suggest something similar. I know that it can be really helpful to go with the child's flow. It takes creativity, but if you can blend bits with another activity or connect the topic to something else of interest, it may help.
3) consider changing the time of day that you work with your child- find their peak time for being calm and focused (if any!) and work in bits then
4) perhaps a change of scenery would work? Choose a spot in the house that is special just for learning and go there each time... even if it's a certain chair or something. My daughter knows that we nurse in the same chair all the time and knows it's time to settle and eat when we are there... otherwise, if I try to feed her somewhere else, she gets very distracted. this might apply to learning too, particularly taking care to choose a place with minimal distractions (i.e. not the toy/playroom,
!)
5) will your older child "set the example" where you can ask your 18-month old to copy? Maybe saying "watch! can you do this/that?" (The older child could sit still and watch you, then ask the younger one to do the same).
Okay, I guess I'm out of ideas for now. I'll try to get back to you if I think of anything else. I hope this helps a little. Good luck!