You have to use the global variable trick, or accept use save-as-a-promise trick.
var getStuff = $.when(req1,req2).then(function(data1,data2) { return data1.concat(data2); });
//the variable getStuff is now a promise and any .then chained
//to it will have data1.concat(data2) passed to it as an argument
getStuff
.then(function(data1Data2) {
console.log(data1Data2);
});
//the next time you want to use it, you have to use the same promise-interface with .then
getStuff
.then(function(data1Data2) {
console.log(data1Data2);
});