I have also had consistent issues with snap pea germination, so I have started germinating them indoors in a wet paper towel tucked inside an (unsealed) ziplock bag. This seems to give them a leg up. I get around 90% germination, then I plant the successfully germinated seeds. It helps even more if you can first transplant them into small pots (like the 6 cell plastic packs that you would get with plants from the nursery). If you re-use cell packs, make sure they are washed first - I use a 10% bleach solution to soak for 10-20 minutes, then rinse well). I put the planted sprouted seeds in the cell packs in a bright sunny area out front of the house during the day, then take them in at night. After they are 2-3 inches high I transplant to the garden plot with a little fertilizer or compost worked into the soil. I have had the best luck transplanting in early April (in Pennsylvania) and covering with a light row cover. They are fairly frost resistant, so the row cover is not absolutely necessary, but they grow quite well under it and are protected from critters and strong winds. At this time, I just put some small sticks under the row cover for them to begin climbing on. When they are about 8 inches high I take off the row cover and erect a trellis along the seedling row for them to move onto after they outgrow the small sticks.
Growing peas in the heat of the summer doesn't usually work well, and to plant in the fall, you have to time it right. See how long the seed packet says it takes for them to produce fruits. In the past, I have tried planting in late August, but by the time they started setting flowers, frost was starting to occur. While the plants are resistant to frost, the flowers and fruits are very sensitive to it and frequently shrivel and fall off even if the plants are covered with a row cover to protect them. Therefore, you should try to plant your second crop so that flowering and fruiting occurs well before frost is expected for your area. If done right, they will flower and fruit for at least 2-3 weeks.