what jsreport version are you using? i've done a test with latest jsreport (4.7.0) and it works.
using this data
{
"data": [
{
"second": {
"records": [{
"first": "Boris",
"last": "2",
"value": 1
}, {
"first": "Matos",
"last": "22",
"value": 18
}, {
"first": "Jan",
"last": "12",
"value": 10
}, {
"first": "Blaha",
"last": "34",
"value": 8
}]
}
},
{
"second": {
"records": [{
"first": "Pablo",
"last": "48",
"value": 49
}, {
"first": "Juan",
"last": "92",
"value": 16
}, {
"first": "Martin",
"last": "45",
"value": 17
}, {
"first": "Jhon",
"last": "10",
"value": 7
}]
}
}
]
}