When the chuck operator (=>) is used with UGens like `foo => bar`, it pipes the audio signal from foo into bar. Some UGens accept input (like dac, although dac is maybe more properly an audio sink than a UGen), some produce output (like SinOsc), and some do both (like Gain). I believe oscillators like SinOsc do not accept input, so `SinOsc foo => SinOsc bar` wouldn't make sense. This program produces no audio, for example, at least AFAICT:
SinOsc s => SinOsc t => dac;
400 => s.freq;
600 => t.freq;
1::day => now;
Your first example creates two sine oscillators and sends their output to the dac to play. You'll need to set a frequency for the oscillators and advance time to hear anything, of course:
SinOsc s => dac;
SinOsc t => dac;
400 => s.freq;
600 => t.freq;
1::day => now;