This is an oft-repeated, but completely inaccurate understanding of what dispensationalism teaches concerning salvation. Some early writers, like Scofield, made statements that when taken in isolation could give the impression that salvation under the Law was different than since the cross. However, that was over 100 years ago. Subsequent theologians, particularly those of the last 60 years have made it abundantly clear over and over again that there is only one way of salvation - ever - namely by grace through faith alone.
I have been a believer for nearly 30 years and have studied and taught dispensationalism for the last 20+ years, being a Dallas Theological Seminary graduate - and personally know some of the most well-known dispensationalists including Walvoord, Pentecost, Ryrie, Toussaint, Zuck, Constable, Bailey, Dyer and others - and I have never encountered anyone who has taught anything but salvation by grace through faith alone in every era of God's program.
If there is a dispensationalist out there who teaches two ways of salvation or dual covenant theology, he is out-of-step and completely outside the mainstream of dispensational theology.