Me contesto a mi mismo
Ya encontre la solucion al problema que tenia, gracias a faino,
tenia que introduir el estilo float, dejo el codigo tal y como me funciona a mi, sin el script que esta mas arriba
Cita: <form action="dummy" method="post" style="float:left;margin-right:5px;"><select name="choice" size="1" onChange="jump(this.form)"><option value="http://localhost/brossa/?p=94">105x210 mm</option><option value="http://localhost/brossa/?p=96">105x297 mm</option><option value="http://localhost/brossa/?p=98">120x120 mm</option><option value="http://localhost/brossa/?p=100">148x210 mm</option><option value="http://localhost/brossa/?p=102">210x75 mm</option><option value="http://localhost/brossa/?p=104">210x297 mm</option><option value="http://localhost/brossa/?p=1">52x74 mm</option><option value="http://localhost/brossa/?p=86">85x55 mm</option><option value="http://localhost/brossa/?p=88">100x210 mm</option><option value="http://localhost/brossa/?p=90">105x105 mm</option><option value="http://localhost/brossa/?p=92">105x148 mm</option><option value="http://localhost/brossa/?p=145">74 x 105</option></select></form>
<form action="dummy" method="post" style="float:left;"><select name="choice" size="1" onChange="jump(this.form)"><option value="http://localhost/brossa/?p=94">135</option><option value="http://localhost/brossa/?p=187">250</option><option value="http://localhost/brossa/?p=199">350</option></select></form>