fresh block

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
    
    line {
  stroke: #ccc;
}

circle {
  stroke: none;
  stroke-width: 40px;
}

.nodes1 {
  fill: grey;
}

.nodes2 {
  fill: blue
}
  </style>
  <script src="d3.js"></script>
</head>

<body>
  <svg height="200px" width="200px"></svg>
  <script>
    var nodes1 = [{a: "a"}, {a: "b"}, {a: "c"}]
var nodes2 = [{a: "d"}, {a: "e"}, {a: "f"}]
var links = [{
	source: nodes1[0],
  target: nodes1[1]
}, {
	source: nodes1[2],
  target: nodes2[2]
}, {
	source: nodes1[0],
  target: nodes2[1]
}]

var s = d3.select('svg')

var tick1 = function() {
	s
  	.selectAll('circle.nodes1')
    	.data(simulation1.nodes())
    .attr('transform', function(d) {
      return 'translate(' + d.x + ',' + d.y + ')';
    })
}
var tick2 = function() {
	s
  	.selectAll('circle.nodes2')
    	.data(simulation2.nodes())
    .attr('transform', function(d) {
      return 'translate(' + d.x + ',' + d.y + ')';
    })
}

var simulation1 = d3.forceSimulation(nodes1);
var simulation2 = d3.forceSimulation(nodes2);
var simulation3 = d3.forceSimulation(nodes1.concat(nodes2));

simulation1
  .force('collision', d3.forceCollide().radius(40))
  
simulation2
  .force('collision', d3.forceCollide().radius(40))
  
simulation3
	.force('link', d3.forceLink(links))
  .force("center", d3.forceCenter(100, 100))
  .force('xposition', d3.forceX(100).strength(0.1))
  .force('yposition', d3.forceY(100).strength(0.1))

s
  .selectAll('circle.nodes1')
    .data(simulation1.nodes())
  .enter()
    .append('circle')
    	.attr('class', 'nodes1')
      .attr('r', 10)
      
s
  .selectAll('circle.nodes2')
    .data(simulation2.nodes())
  .enter()
    .append('circle')
    	.attr('class', 'nodes2')
      .attr('r', 10)
      
s.selectAll('line')
		.data(links)
  .enter()
  	.append('line')

simulation1.on('tick', tick1)
simulation2.on('tick', tick2)
simulation3.on('tick', function() {
	s.selectAll('line')
    .attr("x1", function(d) { return d.source.x; })
    .attr("y1", function(d) { return d.source.y; })
    .attr("x2", function(d) { return d.target.x; })
    .attr("y2", function(d) { return d.target.y; });
})
  </script>
</body>