It is easier to use wait steps in a regular campaign flow. It may be possible to use multiple streams and bounce leads around, but it'll be messy.
See my review of this issue here:
http://www.slideshare.net/jdavidhill/marketo-engagements-and-drip-nurturing-boston-user-group-dec-10-2013
Following....trying to solve this myself using an Engagement Program instead of a regular drip but would love to bounce ideas off of someone if the way I've setup it doable or not. Here's my post: Engagement strea cadence for day 1, day 5, day 10, etc
I had this exact challenge and I solved it by creating Triggered Smart campaigns. Filling out the form on our website is the trigger that starts the flow.
Here's how I set it up:
We have three free trial options so I created three smart campaigns with associated emails and forms. Only email #1 is different and 2-5 are identical for all three campaigns.
Here's the "Flow"
1. Send email #1
2. Wait 1 hour
3. Send email #2
4. Wait 2 days
5. Send email #3
6. Wait 3 days
7. Send email #4
8. Wait 2 days
9. Send email #5
I was frustrated with the inflexibility of their traditional "Nurture" campaign set up but this works exactly how I want it to. Hope this helps!