Epoxidation with peroxyacids
This source states that MCPBA gives preferential epoxidation of the more substituted alkene. This source explains that due to intra-molecular hydrogen bonding in solution, the high degree of polarisation results in an electrophilic oxygen atom that adds to alkenes. The most substituted alkene will be the most electron-rich alkene and hence the most nucleophilic. This is supported by the data shown here and here 4 on the relative rate of reaction of unsubstituted/mono/di/tri/tetra subsituted alkenes.