You can use the Variable Flow Percentage Flow Splitter in InfoSewer to model overflow in a node when the two stage solution in InfoSewer does not have a full pipe in the 1st stage of the solution, http://www.swmm5.net/2011/06/infosewer-link-and-head-calculations.html The problem is that the 1st stage of the solution has non full pipes but the 2nd stage in which the flows are calculated has adjusted depths that show the pipes are full and the HGL is high. The flow splitter is used to allow the overflow water from the high HGL to flow into the overflow link to the outfall.
You can do this by having two Flow Split Curves, one for the low flow to the downstream node and one for the overflow to an outfall. The X value of the flow curve is the inflow to the Loading Manhole, the Y value is the flow in the conduit that you desire to have a flow split. Each of the pipes associated with the loading manhole has a different flow split curve.
Here are the steps in the modeling suggestion:
1. Have two curves: (1) One for low flow and (2) One for Higher Flow (Figure 1),
2. You can set up the curves to have all of the loading flow go to one or the other link as shown in Figure 2, when the inflow to the loading node is below 5 gpm the flow will go to the downstream link, when the flow is above 5 gpm all of the flow will go to the overflow link to the outfall.
Figure 1. Two Variable Flow Percentage Curves
Figure 2. The divided flow in the normal link and the overflow link.