Sure looks that way, at least for the beta. On the Usage section of T-Mobile, the Data tab on my duplicate line says the following: "For duplicate lines, please refer to the voice line that the shared line is connected with for usage details."
It's limited to the tethering portion of your highspeed data plan
Also the DIGITS SIM cards aren't really meant for being used in tablets as they are about getting your talk & text on another device which the iPad isn't capable of doing natively..
If you want to use DIGITS on your iPad, download the app...
I will look for the topic that covers the answer to this question so that I can provide the correct wording. But according to tmobile the duplicate line will share the min/text/data from the main line.
here is the topic link
Replicate your primary DIGITS for use on a second device. Get a duplicate SIM, which is a second SIM card that is a replicate of your existing SIM card. You can put this SIM in any compatible GSM wireless device and it will act as your primary DIGITS. All calls, text, and data are shared between your original device and the new device with the duplicate SIM. For example, get a duplicate SIM for your Samsung Gear 2 or Gear 3, and when you go running your Gear 2 or Gear 3 will receive all your calls/texts. Plus you can stream music without worry of data usage with T-Mobile ONE unlimited date plan.