Hi Anne,
Yes, you could take both caps at once, many studies have been done with a 600 mg dose and it is safe and effective.
If you have never taken this ashwagandha, you may find the effect quite strong at first, if you are sensitive, and taking one capsule at a time can provide a more gradual introduction option, but it is not required. If you do end up feeling a bit sleepy during the day, which could happen at the outset, please take care if you must drive or do something similar.
There are no cautions for taking this with either thyroid medication or with Ventolin, and it is not necessary to discuss with a doctor since neither are considered sedatives, and neither are for treating psychological disorder.
Please read the cautions as there are some circumstances where you may still wish to tell your health care practitioner that you are using this and discuss it with them, for example, if you use this for sleeplessness that persists or if you decide you'd like to continue to use this in a longterm ongoing manner.
For example, some studies have found KSM-66 ashwagandha to support healthy thyroid hormone levels in those who tend to be low, suggesting that if you use this long term, you may wish to monitor your medication levels in the event that your dosage could be adjusted.
All the best,
Flora Health