If you add text-anchor="middle"
to each tspan
you will center them (you have to remove the space between the tspans as well, otherwise the extra space will be considered as part of the first line and they won’t be completely centered).
For example:
<svg>
<text y="50" transform="translate(100)">
<tspan x="0" text-anchor="middle">000012340000</tspan><tspan x="0" text-anchor="middle" dy="15">1234</tspan>
</text>
</svg>
See: JSFiddle