Why are high levels of epinephrine and low levels of cortisol signs of stress?
So it seems to me that signs of stress would be high levels of both of these things.


Yes. Wikipedia claims [1] the same:


  Stressors [...] activate the HPA axis, though via different pathways. [...] Stressors that are uncontrollable, threaten physical integrity, or involve trauma tend to have a high, flat diurnal profile of cortisol release (with lower-than-normal levels of cortisol in the morning and higher-than-normal levels in the evening) resulting in a high overall level of daily cortisol release. On the other hand, controllable stressors tend to produce higher-than-normal morning cortisol.


The same on a webpage of Society for Endocrinology [2]:


  In addition, in response to stress, extra cortisol is released to help the body to respond appropriately.


Yet, there are conditions in which stress can lower cortisol levels. One of them is psoriazis [3]:


  [...] patients with persistently high levels of stressors seem to have a specific psychophysiological profile of lowered cortisol levels and may be particularly vulnerable to the influence of stressors on their psoriasis. 


And some psychiatric disorders [4]:


  Several stress-associated neuropsychiatric disorders, notably posttraumatic stress disorder and chronic pain and fatigue syndromes, paradoxically exhibit somewhat low plasma levels of the stress hormone cortisol.




References:


Wikipedia contributors, "Hypothalamic–pituitary–adrenal axis," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Hypothalamic%E2%80%93pituitary%E2%80%93adrenal_axis&amp;oldid=619115237 (accessed August 7, 2014).
Society for Endocrinology. Hormones. Cortisol (2013). Available from http://www.yourhormones.info/hormones/cortisol.aspx (accessed 07.08.2014)
Evers AW, Verhoeven EW, Kraaimaat FW, de Jong EM, de Brouwer SJ, Schalkwijk J, Sweep FC, van de Kerkhof PC. How stress gets under the skin: cortisol and stress reactivity in psoriasis. Br. J. Dermatol. 2010 Nov;163(5):986-91. doi: 10.1111/j.1365-2133.2010.09984.x. PubMed PMID: 20716227. 
Yehuda R, Seckl J. Minireview: Stress-related psychiatric disorders with low cortisol levels: a metabolic hypothesis. Endocrinology. 2011 Dec;152(12):4496-503. doi: 10.1210/en.2011-1218. PubMed PMID: 21971152.