var urls = ['url','...'], data = [];

var buildRequest = function(url) {
    if (url) {
        return $.get(url);
    }
};

var request = function() {
    if (urls && urls.length) {
        data.push(buildRequest(urls.shift()));
        data.push(buildRequest(urls.shift()));
    }
    if (data.length) {
        $.when.apply(this,data).then(function(res1,res2){
            //TODO
        }).then(function(){
            data = [];
            request();
        });
    }
};

request();

我一般会这样写

async/await 就是基于 Promise 的，但是更方便：

function run(urls) {
    let tmp = [];
    let datas = [];
    let res = urls.map(async url => {
        tmp.push(url);
        
        if (tmp.length === 2) {
            let data1 = await getData(tmp[0]);
            let data2 = await getData(tmp[1]);
            datas.push(data1, data2);
            tmp.length = 0;
        }
        
        return datas;
    });

    return res[res.length-1];
}

async function getData(url) {
    let data = await $.get(url);
    return data;
}

这样是不是可以？

var urls = [];
    var results = [];
    
    for(let i = 0 ; i < 5 ; i ++){
        var url1 = urls[2*i];
        var url2 = urls[2*i + 1];
        
        var p = new Promise(function(resolve,reject){
            ajax(url1,function(res1){
                resolve(res1);
            })
        });
        var q = new Promise(function(resolve,reject){
            ajax(url2,function(res2){
                resolve(res2);
            })
        });
        
        Promise.all([p,q]).then(function(data){
            results[2*i] = data[0];
            results[2*i + 1] = data[1];
        });
        
    }