My suggestion would be to put them both to bed at the same time, perhaps compromising at 7:30 p.m.
As you know, it is a process. Why go through it twice?
Create a bedtime routine that includes both children. They can bathe together, put their jammies on together, brush teeth together (your 8 month old can sit on your lap in the bathroom while you assist your 3 year old with brushing her teeth). All the while discussing teeth and any other silly thing to keep them on task with taking care of their bodies.
From there you can make a big deal out of getting into bed for storytime. Get cozy with them in bed to read a bedtime story. A time for all family members to let down (even if it's just for 20 minutes until your dinner dishes call you back to the kitchen). The younger child enjoys a night time feeding and the warmth of your bodies while hearing your voices. This is the best recipe for drifting off into lalah land as the story or stories continue. This will leave you with the one-on-one time that you and your older child have always enjoyed as you continue reading.
If they are given the same comfort and guidance with the same schedule it should fall right into place for you.
This is the perfect time to establish this pattern for yourselves. As your younger child transitions from infant to toddler she will observe her big sister routine and grow to learn that it is hers as well. It will definitely establish a special bond that will keep bedtime fun for years to come.
Even if the younger child is not fully asleep she will melt at the sound of your lullying voices. The younger child can be placed into the crib at the end of your story(ies). Once you kiss them each tenderly and slip off to bed you should be golden.
Try not to try too hard. Relax and just let it happen while establishing clear guidelines. No bouncing up for a drink of water, 10 more books . . . A simple matter of fact attitude, "This is what we do at bedtime" should be all that you need to say (along with whatever special thing you do to close bedtime). I remember having to work with the younger child not to disturb the older child from time to time but she will get it. Be consistent. Trust me, the effort that you put into this now will lay the groundwork for effortless bedtimes in the future.
Our children are now 8 and soon-to-be 5 (three years and 2 months apart) and this routine continues to work for our family style. We love bedtime!
Sweet Dreams from a Fellow Mom Who Cares!
K.