Namaste,
The idea of two truths is ultimately an idea that is as old as the upaniShads themselves. The idea that effects are simply names, i.e constructs of speech, and the only reality is the cause, is found in the sixth chapter of the ChAndogya (6.1.4 - 6.1.6) वाचारम्भणं विकारो नामधेयं मृत्तिकेत्येव सत्यम्. Shankaracharya in his bhAShya explains - "कथं मृत्पिण्डे कारणे विज्ञाते कार्यमन्यद्विज्ञातं स्यात् ? नैष दोषः, कारणेनानन्यत्वात्कार्यस्य । यन्मन्यसे अन्यस्मिन्विज्ञातेऽन्यन्न ज्ञायत इति — सत्यमेवं स्यात् , यद्यन्यत्कारणात्कार्यं स्यात् , न त्वेवमन्यत्कारणात्कार्यम् । कथं तर्हीदं लोके — इदं कारणमयमस्य विकार इति ? शृणु । वाचारम्भणं वागारम्भणं वागालम्बनमित्येतत् । कोऽसौ ? विकारो नामधेयं नामैव नामधेयम् , स्वार्थे धेयप्रत्ययः, वागालम्बनमात्रं नामैव केवलं न विकारो नाम वस्त्वस्ति ; परमार्थतो मृत्तिकेत्येव मृत्तिकैव तु सत्यं वस्त्वस्ति ॥"
As the topic being discussed is the knowledge of everything through the knowledge of one, and the example of the "clay and pot" is cited to illustrate this, the interpretation given is that all effects are nothing but the causes, and by knowing the causes all effects are known. In fact, there is no different object corresponding to the effect - it is merely the cause with a different name. This is later extended to say that every cause is only intermediary, it itself being an effect and thus nothing but its cause, with a different name. Thus if one were to seek the cause of everything, one is led to the ultimate reality is the Cause, सत्. ( Ch 6.8.4 तस्य क्व मूलꣳ स्यादन्यत्रान्नादेवमेव खलु सोम्यान्नेन शुङ्गेनापो मूलमन्विच्छद्भिः सोम्य शुङ्गेन तेजो मूलमन्विच्छ तेजसा सोम्य शुङ्गेन सन्मूलमन्विच्छ सन्मूलाः सोम्येमाः सर्वाः प्रजाः सदायतनाः सत्प्रतिष्ठाः) - Thus everything in creation is the one ultimate reality with a multiplicity of names and in order to know everything, one needs to know only that one thing, as there is only one thing in reality.
Thus the two truths, one a conventional reality corresponding to the manifold objects perceived conventionally and the ultimate reality that there is only that one, non-dual entity(ekamevAdvitIya) is an upadiShic concept, and not an idea that advaita borrows from Buddhism.
However, we know that this charge has been made since at least Gaudapada's times, for this leads him to comment at the end of his kArikas, नैतद्बुद्धेन भाषितम्. As Shankara explains in his kArika bhAShya, ज्ञानज्ञेयज्ञातृभेदरहितं परमार्थतत्त्वमद्वयमेतन्न बुद्धेन भाषितम् । यद्यपि बाह्यार्थनिराकरणं ज्ञानमात्रकल्पना च अद्वयवस्तुसामीप्यमुक्तम् । इदं तु परमार्थतत्त्वमद्वैतं वेदान्तेष्वेव विज्ञेयमित्यर्थः ॥ The ultimate reality being devoid of the differences of knowledge, known and knower has not been spoken of by the Buddha. Even though the refutation of external objects as superimpositions on consciousness has similarity to the non-dual reality, that this non-duality is the ultimate reality can be known only through the upaniShads.
Regards,