@classname: ~'@{varone}_@{vartwo}';
(or same with double quotes) is the correct syntax and works in all conformant Less compilers. I.e.:
.createClass() {
@varone: one;
@vartwo: two;
@classname: ~'@{varone}_@{vartwo}';
.testClass_@{classname} {
padding: .5em;
}
}
.createClass();