Hi Ana,
Short answer: use a participant attribute to keep track of which flow to transfer back to.
Long answer:
Call.CalledAddress is the address called to enter the executing flow, i.e. in your 2nd flow this would be the name/id of the current flow instead of the name/id of the 1st flow. Call.CalledAddressOriginal is the address called to enter the system, i.e. the phone number the caller dialed, which is actually for the IVR that directed the caller to the 1st flow. I tested and transferring to Call.CalledAddressOriginal in my 2nd flow did successfully take me back to the 1st flow. But I'm actually going to recommend the other option of using an attribute to get back to the previous flow for 2 reasons.
1) The Call.CalledAddressOriginal approach only works with 2 layers of flows. If you transfer from flow1 to flow2 to flow3, the caller would go from flow3 back to flow1 instead of flow2.
2) By transferring to Call.CalledAddressOriginal, the IVR will evaluate the schedules again to make a decision as to which flow to transfer to which could result in the caller getting to a different flow if they've been on the line for more than a minute.